And me being, well, me... I have actually looked into it for them (especially since some of them were my sister) and there are some very cute-looking preschool curriculums out there, some based on picture books, some on bible verses, some on crafts. There are online programs and books and pinterest and all sorts of stuff. I'm sure they would be adorable and that kids would enjoy them and if you've got the time and the inclination, have at it. If you want that email, let me know and I'll forward it to you.
But the short answer is: none. I don't think you should be using any preschool curriculum with your kids. I didn't. I don't. And if you're anything like me, you'll never be as free as you are right now to get out of the house and do fun stuff and be spontaneous with your kids.
Before I had school-age kids we had the freedom to do all sorts of stuff that really isn't possible for us now. We had memberships at the zoo and a kids' museum and we used to go out and do things a couple of mornings per week. Last year I took Gus (he was 4) to visit my Grandmother and she suggested we go to their kids' museum. He was totally blown away by it. Because he had never been to a children's museum in his life, or in his memory anyway.
Because things change, families change. Either your kids will be in a traditional school and you'll have the full-time job of getting them back and forth and fundraising and volunteering (more on that here) or you'll be homeschooling, which I find to be a more relaxing lifestyle, but there are daily responsibilities that must be fulfilled. And as much as I'd like to, there just isn't much time left in the week for the beach and the train museum.
Whatever the word is for the opposite of an unschooler, that's me. We have uniforms and the pledge of allegiance and calisthenics. I like order. But not for preschool. In my house, the preschool years are for fun and messiness and experiences. Workbooks and phonics can wait. Remember when the whole point of kindergarten was for getting ready for school? So what's all this about kindergarten-readiness? If a school I was looking at even mentioned that to me, I'd keep looking. Or just homeschool. No wait, that's what I did.
Some of my kids did attend a two-mornings-per-week preschool before the driving for that started interfering too much with our school day (plus, there are now plenty of people to play with who live at my house). When I was looking at different preschools my number one requirement was that it be completely non-academic. Number two was that it have stuff like a big bin of rice with little toys in it that kids love but I would not want in my house.
I feel the same way about homeschool preschool. I want my preschoolers playing outside and listening to stories and dressing up and helping me in the kitchen and singing silly songs in the car. They will learn to read, and count, and identify colors eventually. Some of mine have been much farther along that path by first grade than others, but all of them have been able to read easy chapter books by the end of second grade and I'm fine with that.
But unless I end up with a fifteen year gap between kids (doing math, no not possible) I will never again have those years like I had with my first three, when we DIDN'T have to sit around the house doing school, and don't even get me started on how sports affect things (that's another post). Don't get me wrong, those years are also absolutely the hardest to be a mother, the years with all little ones. But I'm grateful I didn't complicate my already difficult days by trying to "school" my kids. Because I don't think they need it.