Try Wildcatter Ranch out by Possum Kingdom Lake. For accommodations, they have a bunch of options depending on party size and budget, ranging from a luxurious house to individual cabins to hotel rooms. There are all kinds of activities, some even OK for kids under 5, like feeding the longhorns each morning. Or just chill out with the great views and peaceful grounds. Also, the onsite restaurant is quite good - mesquite grilled steaks and world-class CFS.
If not there, I'd look to the Texas coast. Get adjacent beachfront houses in Galveston or Port A. Maybe Condos in South Padre, but that's a haul with little kids from DFW.
EDIT: With regard to accommodations at Wildcatter, the house rental is self explanatory. The cabins are arranged in groups of 4, with each group next to each other, so easy congregation during nap time. The hotel is the least glamorous, but it has a TON of communal space within eyeshot of the rooms. There's also a cabin set out from the rest, but it wouldn't work for a group as large as yours.