I also highly rate the new town of Seabrook. You can find a review I wrote about my family reunion there this summer on tripadvisor by searching "seabrook, wa"
It is a perfect spot for all ages. We had several houses near Crescent Park within Seabrook which is a great place for all ages to hang out. There will be a swimming pool completed at this park by next summer.
About a third of rental homes have bedrooms on the first floor which is great for the older folks. They don't have a map on their website, but the reservation office could send you one, to assist in picking homes that are close together. Also, some homes have carriage houses behind them, which are a great way to get an extra room with minimal added cost.
We had about 30 adults and 10 kids, and it worked out to around 70$ per night per adult, and half that amount per kid (with all taxes & fees included). So about the cost of an in city hotel room, but you get so much more.
I didn't reserve homes until February of this year for July, and there was still a good selection left, although that may change as the place gets more popular, and closer to completion. There are about 100 homes built now, about half rentals. There will also be a small grocery on site by next summer as well, but if you stay here you may want to make a shopping stop at the TOP Foods in Aberdeen on your way out (about 40 minutes from Seabrook) to do your major shopping. There are also a few small markets in Pacific Beach two miles away from Seabrook.