it is absolutely fantastic. we live near there & go pretty often. ds is 3 & could spend hours & hours there! the sandpit is particularly popular. a word of warning though, we went earlier this week & it was hell on earth because of half term. any chance you could go next weekend?
Definitely recommend it--DS has been a regular visitor since he was about 9 months old-- although as people have already posted, the cafe really does let them down. If you are going during the week there is a nice cafe virtually next door to the swimming pool directly opposite the museum.