Look you can post this same question on here and get a better idea if my answer is not pretty much on the mark
My answer is if you pay a lawyer or go back to family court with this in mind you are spinning your tires. Until they are eighteen the mother is still the one providing for their care. If the one that is not living with your mother at the moment is in Cairo Egypt or something maybe yes but your ex can always say she is visiting and coming back. If the other is pregnant and going or not going to school it is a non issue. Your ex is still providing for both of them and I am fairly certain you will not prevail in family court to reduce your child support payments. Been there, done that, got the T shirt and yes, it sucks.
They will all be listed in the yellow pages of the phone book, with big adds indicating that they specialize in personal injury cases. Most will offer a free consultation, so look them up and get dialing.