It is a big jump for you as individuals, but it's not a big jump as if he didn't see them at all. He does know them, yes it's different, but it's just a change, they're not losing either of you.
Honestly, I know it's REALLY hard, but you need to think, worst case (for you, but it's likely), he'll get 50/50. How will that work, are there any preferences you want to ask for?
My sister has set days over a fortnight, so each parent gets a weekend. They have set nights Mon/ Tues/ Thurs, I think, so the kids get to go to activities on those nights with whichever parent whose night it is. Then weds/ Fri/ sat/ sun alternates each week.
School holidays stock to the routine, except for set weeks each year for holidays.
Christmas/ birthdays are alternated.
Care for each parent starts at a set time each day, so you need to decide, if illness strikes, who stays off, the parent who has them at 8am, or the parent who has them from school collection?
Same for childcare fees, is it the person who's dropping them off who pays for that day, or who's collecting them?
Please get this as part of the contract, so you don't end up being responsible for paying more than 50%.
Expect him to be a dick- to apply for 50% of the child benefit if he can, to try and wangle out of payments. Try to think of every way he can come at you, and cover it in the contract.
A good scenario out of this might be that he gets the 50/50, and decides he doesn't want it. It can be reviewed after a year (I believe that's the regular time), so if he lets them down or they are struggling, make notes for going back then.
I am sorry you're going through this, I just do think going in prepared for this is the best- you don't want to be the one on a back foot.