The courts generally consider preschool as daycare. The cost of which will be split consistent with the income shares proportion (if you're familiar with that). So your question about whether it is considered an extraordinary expenses really doesn't impact child support because childcare is always in addition to child support just like the extraordinary expenses would be.
When considering your options, you may want to think about a few things. 1.) the court may not view kindly on the fact that your boyfriend is asking his girlfriend to watch his kids. I don't know either of you, I'm just telling you this because I've personally watched a judge jump all over a father for having his girlfriend live with him and his kids. He was old fashioned. That being said, both parents have the freedom to make the decision they feel is best for their kids. 2.) If your bf doesn't agree to the new arrangements or would like to propose something else, I would encourage you to put it in writing and send a certified letter. If you don't, it will be "he said/ she said" in the court room.
Personally, I think preschool is a good thing for kids. It helps them learn to interact with other kids with more supervision before they hit kindergarten. I agree with you maybe wanting to keep preschool to only part time, but I don't know that I agree that keeping the kids home is the best. Preschool is expensive, but its temporary and usually beneficial.
