BMX bikes are a different beast entirely. Likewise their BB's are slightly different and generally come in three flavors (size/type). Don't let the small frame and toy like looks fool you. If you haven't already, I'd recommend you search specifically for BMX bottom brackets. Then look at the differences of the BB's and decide which you should have gotten along with cranks too.

Also realize that between the General Cycling Discussion , Bicycle Mechanics , BMX sub-forums, I feel you picked the least relevant and least perfect place to post this question.