A few thoughts.
First, who are the kids involved? Are there any computer games being played by any of these kids which have a Cold War theme? I know difficult child 3 plays a couple of computer games which are like this. I also know that kids often then act out scenes form these games, or make up their own scenarios based on themes or characters in these games. difficult child 3, for example, uses a favourite character as his code name or avatar in a number of forums. If we go bowling, for example, he will use tis same code name on the core sheet, like his own personally-chosen nickname.
The problem with computer games (or any other kind of intense gaming) is when the lines between game and reality gets blurred. The problem isn't the game, it is that some people have much more difficulty making the distinction between fantasy & reality. At some level, all te kids will happily join in playing a game like tis - to a point. But ten for the other kids, the point at which the kids can say, "Enough!" can be difficult to define or determine.
I think it is quite likely, if tis game is not primarily sourced form your own kids, that the other kids need some guidance in what is acceptable and what is not. Kids are not noted for their ability to consider all aspects of an issue, and can very easily "lose the plot".
Think about the similar (at some level) situation of an adult couple who like to play bondage games in the bedroom. Before doing something like this they need to set up ground rules - some way of signalling when someone has had enough, wants to stop. There needs to be true freedom to say, "Enough!"
Now with adults, we have that extra maturity that makes it possible to recognise, even in the heat of passion, when we have gone too far. We are more likely to consider the welfare of the other person, to maintain that link and consideration. But children o not have that experience and, moreover, are far more self-centred when their own amusement is concerned. It is too easy for a game like this to go bad. Especially the kid who is the main stimulus for this (and I believe it is one child above all others, a more controlling child too, possibly an older one but definitely the most influential one who is behind this). The degree of detail in tis game, such as needing to physically restrain the 'hostages' instead of pretending to, worries me. There is an unhealthy need to actually physically control and restrain and it could be the beginning of a sexual perversion. OK, a little play now and then is probably harmless. But if tis game is being played constantly, it is unhealthy.
Also unhealthy is the tickling. It is just one more example of control and manipulation. It is bad for the girls to be made to endure this (to be made to believe that it is acceptable to have to endure this) and for the boys to think it's OK to do tis. Rape is not about sex, it is about control. It is about degradation. By enduring this kind of thing, the girls are being taught that it is OK to put up with this. That it is acceptable behaviour.
Role play games are perfectly OK, including spy games like this, if at any time (such as when a parent calls, "Time to come in and wash for dinner!" the child can immediately stop the game and comply. If they want to play these games, each child can PRETEND to be tied up; can pretend to be tortured or to torture; but there should be no element of actual restraint of actual torment. And tickling can be torture for some people. It is unacceptable. Totally off-limits especially out of parental supervision. There should be nothing in the children's play that cannot be played out with a parent watching. If any of the children feel that having a parent watching would make them feel uncomfortable, then the game is wrong.
An example here of the way kids can have a game get out of hand - a local game that mother in law has observed local children playing (about the same age as your children) involves kids taking turns at punching another kid, or being punched. The kid being punched is apparently a willing victim; they do it to see how many punches they can tolerate. That kid then gets the chance (or is required to) punch the next apparently willing victim. A kid who doesn't want to play is labelled a wimp and often becomes a target for bullying to varying levels.
mother in law tried to intervene; she told the kids to stop. The kid punching said, "It's OK; he likes it, don't you Jim?"
Jim nodded with a wide grin, but mother in law wasn't convinced. Jim had to agree, or be labelled a wimp. Privately the kids hate the game but the culture of it continues; failure to play the game labels you in the eyes of other kids. it is circular logic and bad logic; it is self-perpetuating and should not be allowed to continue, by toe adults in the vicinity (it happens as the kids are travelling to and from school on public transport).
I saw similar behaviour when these same kids were younger. A kid who I knew to be a decent kid, the smart kid in the class and generally a really decent, caring kid began to hassle another boy and wrestle him tot he ground to take his backpack off him, then ran off with it. I made them stop and the aggressor said, "It's OK, we're only playing. He knows tat, don't you?"
But although the other kid nodded, I could see he didn't like it. My young friend the aggressor was only acting out what had been done to him allegedly in the name of friendship and "playing a gam" - he didn't like it much either, but it was hat you do if you want to fit in.
My young friend soon stopped this kind of game play when I told his parents, who handled it really ell and actually showed their son the psychology behind how tis kind of bullying is insidious.
So, in short - this kind of game is unhealthy, because first it is hidden; second, they are acting it out a bit too realistically with actual physical restriction and torture (even to a token extent; it is still too far) and also because I suspect behind tis game is one main control freak who could be getting the beginning of some sexual jollies out of this and who needs to be brought back onto the right track and shown a bit of balance.
When I was a kid, I got involved in other kids trying to play games like tis. There were various kids of my acquaintance who wanted to play controlling games and who didn't give me (or other kids) a choice. At the time I was terrified, but afterwards when I tried to complain, I was told that it was only a game, not meant to be taken seriously, and if I was scared by a game ten I shouldn't play it. Not much help when I was given no choice! One time a female friend and I were walking home from school and a neighbourhood kid and a couple of his friends dragged us into an abandoned house (already thoroughly vandalised - I remember a lot of broken glass) and locked us into the bathroom. THese were not kids we played with ever; the ringleader was a local bully we were all terrified of, he was bigger than the others and obviously giving the orders. I can't remember how we got out, I think my friend broke the window and climbed through it, then came round and unlocked the door. The boys were gone but we knew they would be back. At other times I was locked in a toilet block after Girl Scouts when a kid who was sent to make sure nobody was left inside, deliberately locked me in "as a joke" then reported back to the leader that the place was empty. Luckily for me, the door was weak and gave way. I would have been there for hours, possibly all night, if I hadn't been able to escape. Again, I was scared and angry, but the kid was not punished (other than a scolding) ad from the kid's point of view, I shouldn't have taken it so seriously.
Kids in this situation are under a great deal of pressure to conform and to accept the game rules as imposed, or risk losing the respect and friendship of not only the other kdis involved, but the winder community. They don't yet know the rules for what is OK and what is not; games like tis are how they learn, and parents need to step in and reinforce when a game looks like overstepping the boundaries.
So your instinct is right, put a stop to it. Ask them to change the game, or use more imagination and fewer props.
Two rules I suggest you give to the kids:
1) The game must include nothing that a parent would not be OK with (ie no secrecy permitted on he grounds of "my mother would be cross if she knew"); and
2) All participants should have the freedom both physical and psychological to leave the gam at any instant they choose.
Those rules also apply to life.
Marg