Oh I'm so envious - we left Brussels a year ago. Brussels/Belgium is great for children. There is a good French language school at Joli Bois which is used to non-French speaking children. I think all of the English language schools are OK, but they are all very different - you need to decide whether you want a big school with fab facilities like BSB, a cosy school like BISB or a bit of a pre-prep type hothouse like BJAB.
Good things to do - obviously Woluwe Park, and Rouge Cloitre park is also good for children. The ice rink does children's skating on a Sunday afternoon (that's near you - can't think of its name at the moment, but there is a swimming pool in the same complex) with a closed off area and they give them mini zimmer frames to keep their balance.
Indoor play - Planet Kids at Wezembeek, they have a ball park bit for babies. Flying Kids at Wavre is better, but Wezembeek more convenient.
In the summer, try going to Kessel-Lo, near Leuven - huge adventure playgrounds, something for all ages, also a huuuuge outdoor pool there (go early - the Flems don't do anything till lunchtime and you'll have the place to yourself in the morning).
BCT is very helpful and a good way to meet people - you might also check out Xpats.com which is the website for expats - everything from lonely hearts to childcare and toys.
I can't think of anything else at the moment. We lived in Flanders for 5 years and really, really didn't want to leave.