your waiting period doesn't apply to your ftb eligibility. it will however impact the PPS - you can apply for it, and may be granted but you won't get any actual payment until the waiting period is served

as to child support - essentially you have no choice - he has an obligation to his children. but if he is on minimum income, he'll be on minimum CS payments. if you don't apply and collect the CS payments you won't be entitled to max rate FTBA - you'll drop from around 150 per baby per fn to 50 - so it's something you best do. they will assess him fairly on his income - they're not going to tell him he has to pay X amount each month if he doesn't earn that amount!