Janna, my son weighs about 140 pounds.
However, Seroquel is dosed by clinical response rather than weight. D is still on a relatively low dose. Most kids are dosed in the 300 to 600 mg range, although I have seen reports of increased anxiety at 300 mg so you may have to go higher than that. It's worth a shot. If it doesn't work, you can always lower it back down.
We have seen it work on both anxiety and depression in J. I hope it works on D's anxiety as well.