The Mayborn Museum is the perfect place to take your kids during the hot or rainy months in Waco. We took our two year old there, spent five hours, even missed a nap, and hardly noticed the time passing.
On one side of the museum there are exhibits of interest to both children and adults, including a mammoth excavation and replicas of a prairie cabin, wig-wam, and tepee. The other side is aimed more directly at children, with interactive exhibits including live animals, model trains, a butterfly garden, a newsroom, and much more. We didn't even get to the second floor, there was so much to do. But, if you do actually exhaust all the regular features, they have traveling exhibits to keep things new.