Not sure if you know this but Walnut Creek has 2 school districts, Mount Diablo Unified and Walnut Creek unified which has couple of highly rated elementary schools.

The other elementary school option in Northgate is Walnut Acres Elementary. I have a child there and have been happy with the school so far, although the 2nd grade class sizes are huge this year. There is an active PTA and the teachers are generally pretty good. 

Since you are considering private and not sure about going to a Christian school, I heard that Sonder Creek Academy and Seven Hill are both great. If you want feedback about Valley Verde from current families, the Walnut Creek mom group on Facebook could be helpful.