There is no answer to this, only the best you think for your kids.
Firstly your ex should be paying. I would revisit this.
If not, don't remove your son if he is happy & you are happy with the school. With the divorce, being older etc you could make him very distressed.
Check out the state option for your daughter and apply immediately. Buy yourself some time.
Remember you can always change things if it doesn't work out so don't heap too much pressure on your shoulders. Nothing is fixed in stone.
You could send your daughter state til 8 although don't mention it on here as you'll get lynched!
You could speak to your sons school about the situation - they may offer you a significant discount. Reception to Yr3 can be tough years to fill for prep schools, they may prefer to keep her at a discount to keep figures up.
Don't mention school fees In front of your kids. I don't think it's good to chat with them about it anyway, 'we were lucky enough to have a choice to pick the best school we thought for you' is about the limit I think.
Loads of people use a mix of state and private for all sorts of reasons so don't worry about that. I use both, not because of cost but because of moving and finding the best fit for my kids. They don't see it as more money being spent on one or the other, they see it as me trying to make them all happy!
Don't kill yourself to send them both private. Its not worth it and will make you all unhappy.
Personally I would start looking at the state options for my youngest and leave the eldest alone. But do check that he loves it there before agnosising over it!! In 3/4 years your life may look very different so you can reassess then.
Best of luck!