Upstream SoCal was suggested. I actually did a five-day SoCal trip with three nieces and nephew when they were in that age range, maybe four years ago. They LOVED that trip. Between the Warner Bros studio tour, the Getty, a TMZ tour, a Haunted Hollywood Homes tour, eating/touring Malibu, Manhattan and Venice beaches, a day at Disney...we had plenty to do. We have cousins there so we were able to visit with them, too. (Interesting intersection - one of those cousins lives in a house on the Haunted Hollywood tour because of something that happened there before he owned it!) It was a great trip. We stayed at a Residence Inn. While the pool was not resort like, the kids basically had it to themselves which they liked. Plus they could walk into Downtown Burbank, which is relatively small, safe and interesting enough.

I've also gone to Paris and Amsterdam with kids that age. I know, the OP said stay in the US, but those were memorable destinations. The kids loved those trips, still thanking me to this day. The hotels and transportation are the expensive parts of those trips. If points and miles can pay for them, it's easy to tour both inexpensively. The flights are only eight hours or so from the US. Just throwing that out there. BTW, the kids actually liked Amsterdam more than Paris, but love they fact they now have some frame of reference for Paris.