The Blue Valley School District has a peer model program that is very reasonable, but it can be a challenge to get in. Both of my kids did this and it really prepared them for school. They had "reading buddies" with older students and they went to music, pe and art.
Go to BlueValleyK12.org for more info about the district. I'm pretty sure the Olathe district has something similar, but I think Shawnee Mission cut their program (Any moms out there that know if SM still has a preschool program???)
You may also want to look at Creme de la Creme, Barstow, Primrose and the Goddard School. Avila University has a Montessori program that is not religiously based. (My daughter went to Avila for a year before we got her into the peer program. We liked it there.)
If you are out and about, pick up a KC Parent magazine. They ususally have a pretty comprehensive listing of schools. It may be helpful. Good luck!