The Houston Zoo across from the Medical Center is a great day trip for a family of young kids. Ours are 4, 5 & 6 and it is just about perfect. Most of the venues for animal watching are "kid friendly" meaning you will not have to hold them up or prop them up to see. My personal favorite are the giraffe's. I would pack a lunch and in the summer months try to make it early to beat the heat. Parking can be an issue so bring a wagon or stroller in case you have to walk a bit.