Wow you’re really selling yourself short here op. He’s a business owner and can see you coming.
What is right and what is necessary to give your kids the best outcome are different. Personally I would take your half of the 75k and use that to set yourself up.
I would be taking into consideration moving costs, set up costs, furnishings, getting new contracts on things, the consumption of time etc. Don’t underestimate all the hidden costs there.
And what happens if he doesn’t like his 40% and ends up being an EOW dad and starts hiding his income? Business owners can do all sorts. You’ll be paying the nursery fees alone and seriously struggling.
You have provided him with 2 kids and their 60% of the time with you should be in as near as damn nice place as dad’s. His earning potential is clearly higher than yours and it sounds as if he is earning well. He will recoup 37.5k in no time.
All he’s interested in is keeping the family home by the sound of it and seeing what he can get away with whilst spinning a yarn or reasonability.
Get yourself a decent solicitor. They will advise against your plan.