Chapultepec Castle was probably one of my favorite stops on our trip to Mexico City. Sitting up on a hill, it has amazing views and the castle itself is breathtaking! My kids love going to a castle and this one was not any different, even though it was more museum and less exploring (like with the castles they've been to in Europe), they still had a great time. We even heard that the castle is rumored to be haunted and they still wanted to be there :)
Even the walk up to the castle was great! It wasn't too hard and the views from this high point in Mexico City made for a great background. I definitely recommend it!