To you and the OP, I don't understand the logic behind this. The OP's guy is going to be young in K and maybe more immature, and yours isn't really talking much, so you're sending the kids to school EARLY? It makes no sense to me. It seems it would be better for a younger kid (and I say this as one of the youngest in my class, graduating at 17) to be with mom/dad for another year (for the OP), and to be around an adult who will talk talk talk to help the late talker out more than being around other kids who don't have full speaking skills yet.
I would keep kids closer to me, not push them away to the big world, in those situations...(and wish my mom had done so, too, though late talking was my issue, LOL)
I put that out there to think about; I have nothing invested in your kids and don't need to know, but if it makes you think about the alternatives then it's good.