Have a unique question about social security disability and payments to children when multiple mom's.

I got divorced in 2012 and the whole social security disability payment goes to the mom although i have 50-50 custody. Didn't appeal and too late. However, my question is the following: At the time i had three kids on the payment. The payment is like $1,100 whether one has 2 kids or 100. But when the kids fall off when they reach 18 years old, it changes when the total number of kids is below two. So it will be like $550 when one more kid falls off. WIth all that said, my fiance is pregnant. Since the total check goes to my first wife, is she entitled to any of the money now (fiance)? And what happens in 3 years when my other child falls off and i am down to one kid with my ex wife? How does my fiance get half of the $1,100? Hope i have presented the situation accurately.