I agree, this is how I would do it. Half the costs of a full time nursery place per child, per week. You could bung some money in for essentials if you paid them out of your savings too.
I.e 1 child at home with you for 8 months, work out costs of one full time nursery place for that period in your area (this is publicly available info), charge him half.
2 kids at home for 3 months, work out costs of 2 full time nursery places for 3 months and charge him half.
This is money he has saved because you’ve been looking after HIS kids, so he hasn’t had to fork out for childcare.
Another way to do it would be to work out what you’ve “lost” by taking care of his kids (so salary vs what you actually received in mat pay). So if you earn 30k a year, but you got 17k the year you had your kids (mat pay and maybe benefits), charge him half of the difference for each child (so for one child this would be 13k/ 2, so 6.5 k each). This would easily take you over the amount he says you owe in child benefit.
Present him with a fully itemised bill, set 5 k off, allow him a period to query any items and then demand the money or you’re off and he’s going to be on the hook for maintenance and losing 50% of his savings.