I agree with the Caprock Canyons selection, especially if you don't have a boat. There are several other larger lakes in the area, Meredith, McKenise and Greenbelt, but you really need a boat to enjoy them and get to good fishing places.
With Caprock Canyons, you get the added drive time to spend with your kiddo and there are some nice trails in the area along with playground equipment if you get tired of fishing.
Speaking of fishing, it is free in the state park and when you check in at the HQ they'll loan you tackle if you don't already have any. You provide the bait. The Allsups in town usually has worms and canned corn.
Don't forget to fish the upper pond if you are not having any luck in Theo. Most people don't even venture up there and we've had some pretty good luck with it.
Have fun.