We moved to Brussels for 2 years when my son was 1, and my daughter was born there. This was a few years ago so hope my advice isn't out of date.
Two good resources are bctbelgium.org/ and www.expatica.com/be/
The BCT is v much like NCT but as well as doing childbirth stuff they also organise playgroups etc- great for getting to know people. The groups tend to be attended by expats from all over the place, not just the UK, but run in English. Very helpful as you get to know people who have done it all before so can advise you.
Expatica is just a website with info but it does have a good forum- I tend to find the people on there can be a bit grumpy/arsey but they have good info.
Antwerp is lovely but I only know it as a place to visit. Brussels is also great- we lived in Uccle which is French-speaking and great for families (not the cheapest though, although rents certainly much lower than London).
On schools, it's a tricky one as the approach you take will obviously depend on how long you're there for. I would be somewhat reluctant to put an English-speaking child into French or Flemish school if you are only staying 12 months, but if you're probably staying longer/permanently, then this may be the best option. I can't think of an English-language school that isn't fee paying. NB also that it certainly used to be the case that you have no chance of a place at the European School unless you work for one of the Institutions.
There's quite a lot of red tape involved in moving to Belgium- registering with the commune etc- and I'd try to find a good book or similar which sets it all out as it's quite frustrating otherwise. Also get a good accountant- taxes in Belgium are high but there are numerous (legal) deductions which your accountant can advise on.
Overall it's a good place to live and have kids- in particular, local schools and healthcare are fantastic and quality of life is great.