I would fight tooth and nail before a family member of mine was put on effexor. It is one of the hardest SSRI/SNRI medications to take and to stop taking. It does help many people but the side effects are just too much for it to be a sensible choice, esp for it to be the first medication you try.
Not only is weight gain a problem, it is one of the minor problems - at least in my opinion. Effexor has a very short half-life (meaning that the body can clear out half the medication in a very short time) and if you take your dose even one hour late you are likely to feel withdrawal symptoms. You can find a whole list of them by googling "effexor withdrawal". One of the side effects is called brain shivers. It is hard to describe but it literally is like an electric jolt in your brain. It can happen if you turn your head quickly, but movement is not needed to feel it. It can also increase anxiety. I have always been a fairly social person. Never had a problem going anywhere, with or without a group. A couple of weeks on effexor and I was almost unable to leave the house. I was also afraid to be around people, no matter how well I knew them. It was truly awful.
Stopping the effexor was brutal. I felt like I had the flu. Even though the medication leaves the body fairly rapidly the withdrawal still lasts for several weeks. Your body has to get used to not having the medication and it is not a short process.
There are many people, including members here, who take effexor and have minimal problems or the problems go away after a couple of weeks on the medication. Knowing that, I would still want to try quite a few other medications before I tried effexor.
There are non-medication therapies for anxiety. If there is any PTSD component to her anxiety EMDR therapy is excellent for treating it. You can google EMDR therapy to find therapists who use it. Biofeedback is highly effective for anxiety in many cases. One member here bought a gadget called a stress eraser for her son. It is a little rectangular box that you put a finger in. Then you work on relaxing and deep breathing by watching the cues on the little screen. This website has a video that tells more about it.
http://stresseraser.com/stresseraser-refurbished/?gclid=CJaixJyrvaICFRbyDAod3yVX6Q
You can also search for biofeedback devices on amazon.com.
Meditation is another great way to treat anxiety. My husband has used guided meditation for years to help with all sorts of things. He got me hooked on it while we were still dating and now our younger two also love it. Guided meditation has you listen to a voice talk you through various breathing exercises, relaxation techniques and thoughts. It can help you calm down or even go to sleep. I often use the meditations when I am fighting a migraine.
It may sound strange, but does your difficult child ever listen to audiobooks? Many people find them calming. Years and years ago my father read several Uncle Wiggily books onto tapes for my oldest (who was a toddler at the time). Something about his voice is incredibly soothing. It not only soothed my savage difficult child, it also worked on other people. We could not play them in the car except with headphones because they relaxed husband and I too much! Gpa was a jr high teacher at the time and tried the tapes with some of the students in his school who had anxiety and other problems. It was amazingly effective at helping the students to calm down or stay calm. While I cannot send my dad's tapes to you, you could find other audiobooks to try. Mary Pope Osborne reads the Magic Tree House books and they are very soothing to listen to. They also appeal to both boys and girls. (If you are not aware of them, they feature a brother and sister who find a magic treehouse and travel all through time to help make sure the right things happen. They are well researched and your son is at about the right age for them, or at least the age when my youngest liked them.) They are available on amazon in sets. The first 8 books in the series are about $20 on CD from amazon.
You might also look for audiobooks read by the dad from Happy Days. Not sure of his name off the tip of my tongue, but we used to have a couple of audiobooks read by him that were calming to my kids.