We found that even in the wet, Disney was fun. There was enough shelter in most places that you didn't get soaked but I would bring a brolly. We didn't do any shows (not the thing for Mr 4.5) but he loved the Paint the Night Parade. If you can take a break in the afternoon (when its very hot) and come back in the early evening, I would   otherwise its a very long day for little ones. MTR is very convenient.