The "musée des enfants" is a place I discovered fairly quickly when I moved to Brussels. Simply…read morebecause I live almost next to it.
When I say discovered, I should specify the gardens. There is a public playground outside, which is open daily until 1800 if I a not mistaken. There are swings of all sizes, slides, and a few goats (behind a grid) that will entertain any young child.
Now this year we were looking for a location to celebrate my son's 6th birthday, and they offer very nice all inclusive packages (animation, cake and drinks), I know it is available in French and Dutch and strongly believe it is possible in english.
The entertainment is provided by young adults that browse through various rooms of the museum where different themes are on displays. The worlds cultural differences, the cycle of life, etc... the displays are well conceived and the level of detail provided by the leader is adapted to the audience.
The ONLY weak point is that the museum is a real classic for schools in Brussels, so you might be confronted with a group of kids who have already seen everything;-) But they do renew the displays on a bi-yearly basis.