TBH I think that spending time with a parent is more important than nursery. This is the last 6 months before you will both be able to spend an afternoon with older one doing fun stuff and spending quality time together before they start school.
Yes routine and preparing for school is important. But they are going to have that for the next 14 years! And in the mean time they have the rest of the week at nursery anyhow...
If you really won't allow one afternoon of nursery I personally think you are a bit unreasonable. And the nursery don't have to loose the funding, you can ask them to keep the space open and claim the funding as your child may well be using some of the days anyhow if ex decides he can't handl it after all. Nursery loosing funding is really not a consideration in the best interest of your child and so I don't know why you would think it important to your child to be honest.
OK, so assuming you are going to dig your heels in and refuse to allow your child to spend time with their dad then how about suggesting he has younger one for afternoon, they both go and pick up older and then come home after tea. Or he hs younger one drops off before pick up from nursery and then has older one in afternoon.
But he is not going to be able to do quality time things with them, like swimming, park, days out etc if you restrict contact to a couple of hours.
Give him the day and one day at the weekend, insist he sees both children on at least one of those days and has the baby for some time alone too.
Remember that when your child starts school he's likely to get every other weekend from friday to monday with his child anyhow, so you're going to have to get used to it at some point. Embrace it, don't look for the negatives or make it impossibly hard for your ex.
Work together, it will be so much better for your children if you can work something out over this and don't dictate simply because your child needs to get used to the next 14 years in education by being at nursery rather than with their parent.
Sorry I know you didn't want to hear that opinion most likely, but I hope it makes you think a bit more about it.