Copied something I wrote on another thread:
I think most places are reasonably child friendly. Dd was 2 when we moved here and we have taken her in lots of cafes and restuarants without incident, even some quite posh ones. Chez Leon (Rue de Bouchers and at Bruparck)is notably child friendly and kids eat free with their parents.
Things to do:
Definitely have a stroll round the Grand Place and go and be underwhelmed by the Mannekin Pis.
The Museum of Natural History has a fab dinosaur collection and lots of little play areas. The Children's Museum in Ixelles is quite good fun too. Stroll in the Bois de la Cambre. Trip on the metro or tram to Brupark to see the Atomium (better to look at from the outside). At Bruparck there is a food village, a multiplex cinema, a pool complex, playground, merrygoround etc, and Mini Europe which has key European sites in miniature.
The Art Museum and Magritte museum are good, if you like that sort of thing. There is the Parc de Bruxelles up near there which has another very good playground. Comic Strip museum too.
You can also get the 44 tram out through the forest to Tervuren and the African Museum (full of stuffed dead things - so not to everyone's taste - dd loves it!) but the park that surrounds it is lovely and there is a new cafe by the lake.