I'm not sure how things work where you are...and I have learned all too well how very different things can be handled based on your location. However, I can tell you what we had to go through for difficult child 2's first admission...whatever that's worth.
It took a long, long time for me to be okay with having difficult child 2 admitted. When I finally did, I was shocked! The folks at the ER didn't even want to have psychiatric come down to evaluate him because he wasn't out of control or violent right at that moment. (Nevermind that we had to have orderlies help me remove him from the car and get him into the ER because he was still raging, albeit on the downward side of a meltdown by the time we got there.)
In our situation, we actually lucked out. When the ER docs said they were going to send difficult child home? I threw a fit that we hadn't even had a consult with the folks from psychiatric yet. So, to quiet the almost raging mamma, the ER docs did get a psychiatrist to come down to the ER. Luckily, it just so happened to be our new psychiatrist who was on call that night. At that point we had an appointment for our first consult (difficult child 2 was seeing another psychiatrist who was a moron, to say the least) but the new psychiatrist had never laid eyes on difficult child 2. I talked to the new psychiatrist at length that night about his new patient that he was meeting in the ER rather than the office. We decided that since difficult child 2 was calm and lucid by then, as well as half falling asleep, that I would take him home and we would do a planned admission the following afternoon. It gave the new psychiatrist time to line up the team he wanted to come in and run tests on difficult child 2, and was less traumatic for difficult child 2. That's actually when we got the Pervasive Developmental Disorder (PDD) diagnosis.
Since that first admission, it's been alot easier to get difficult child 2 admitted through the ER. Heck, after being admitted 5 times just since February of this year? They just about know us on sight! (How sad! lol.) But I've still had moron on-call psychiatrists that tried to refuse admissions because difficult child 2 wasn't currently in the middle of a meltdown. For those morons, I just whip out my Warrior Mom armor and commence to battling - refusing to bring difficult child home - demanding to speak to superiors - heck, I've even called our family attorney from the ER with a psychiatrist standing right in front of me! lol.
My best advice is to first - find out which hospitals in your area do psychiatric admissions. (Hint - most children's hospitals do have child psychiatric wards.) Then, take your difficult child to that hospital's ER. Demand that someone from psychiatric come to the ER to evaluate your difficult child for admission. Explain that you fear for your safety and the safety of your difficult child. Be brutally honest about what you fear, why you fear it, and be prepared to give details on past episodes, threats, etc. When all else fails...dig in your heels and refuse to take him home if you feel there is a threat to anyone's safety. Don't be afraid to ask for supervisors, patient reps, or anyone else you think might be able to help you make your case for admission.
You can also call your child's psychiatrist or therapist and ask them to make a recommendation for immediate admission. Sometimes it helps if you tell the ER folks that Dr. So-and-so advised you to bring your child to the ER to be evaluated for admission.
Another avenue is to check with your local Community Mental Health. Ask them if they have an 800# for crisis situations. I know our CMH has a crisis line that will help you find a hospital & if needed, will send out their own evaluation team if the hospital you go to doesn't have psychiatrists on call. They will often work with the ER docs to find a suitable placement for stabilization in a crisis situation.
Just understand that you will most likely have to fight to get your difficult child admitted. Be prepared to make your case for admission and DON'T BACK DOWN if you feel there is a real danger that your difficult child might hurt himself or someone else.