Kimbell Art Museum
The permanent collection is free, the building is a Louis Kahn masterpiece, and the galleries are small enough that kids do not get overwhelmed. There is real range here from Egyptian and Asian antiquities to Michelangelo and Caravaggio, so you can play a quick scavenger hunt and be out in an hour. The vaulted ceilings and reflecting pools make the building itself the attraction for a lot of kids. It sits a two-minute walk from the science museum, so on a genuinely miserable weather day you can chain both. Free admission means zero pressure to stay longer than everyone can handle.