Since my husband is Flemish and both kids are being brought up billingual Spanish-Flemish (picking up some French along the way) we have chosen the Flemish-speking education system for both. Baby Pingu goes to a Flemish creche and Bibu goes to a Flemish "basisschool" (kindergarden and primary school). We recently learnt that brothers or sisters of pupils at Flemish-speaking schools born in 2012 had priority to be enrolled at the same school as their big siblings for the school year 2014-2015, provided they became enrolled until Friday 15th November 2013 (end of this week)
There is a website (also evailable in French and English) that explains the whole procedure -also if your kid has no bigger brothers or sisters- on how to enroll him or her to the school year 2014-2015. I have no clue if anything like this exists in the French-speaking education system in Brussels. I could only find this website which provides more info.
One word to the wise, hurry up!