Horizon's by Marriott in Orlando, FL. is one of the best Marriott resorts I've found that meets the needs of young families. There are two large pools, one with zero entry for the toddlers, a water slide, 18-hole miniature golf, tennis, basketball, fishing, and more. We are owners at Horizons because it caters so well to our family.
Besides, Sea World is only blocks away and Disneyworld is about two exits down the freeway.
We have been to the following and my kids (ages 8 and 11) have loved them all
Hilton Head Monarch at sea pines, Surf Watch and Barony Beach. They are all located beach front. Hilton head is a beautiful island with lots of great biking trails. We love to Kayak there. Very low key place with lots of great family restaraunts.
Florida Cypress harbor. Great activities for kids. They couldn't wait to leave the theme parks to get back to the resort
Boston - The Custom House. Not a lot of activities at the resort but the city makes up for that. Fabulous city for kids. The childrens museum and aquarium are within walking distance.
Park City - Mountainside.We went in the summer and my kids had a ball.
Eventually I would like to try them all.