How far from the Chant d'Oiseau area are you? There is Bémel which is maternelle only and has only 4 classes, probably couldn't get much different size wise than Paradis des Enfants.
Chant d'Oiseau is very popular with non Belgians and would normally have space, communal schools were built for extra capacity, but with the rising birth rate, I know they are running out of space there, it used to be 2 classes per year but some now have 3 classes. Heard mainly positive about Chant d'Oiseau.
Notre Dame des Graces is the "sought after" school by Belgian parents, conveniently not far from SC de Lindthout and St Michel secondaires where I imagine the majority of children move on to. It's also a growing school, expanding from 2 to 3 classes per year and has recently completely an extension. But because of popularity with Belgian parents, there is little movement out of the school, once the children start, so the number of non Belgians is going to be mush smaller there. I had a friend who used to have her children there until she moved abroad, she didn't say anything bad about it.
Or Auderghem way, St Julien-Parnasse, know nothing about other than each section is not too big but with secondaire, primaire and maternelle it is a big school, it depends on how the sections are separated physically and on a day-to-day basis. My friend's son goes to a maternelle with primaire and secondaire, I was horrified at first at the thought of little ones in a school of almost 1500 from 2 to 18, but in fact in the maternelle part, it is so enclosed you do not notice the rest of the school.
Then there's an Auderghem communal not too far from St Julien, can't remember the name of it sorry.
I fully understand not wanting to send children to school early, all of my children went only 2 half days per week to a private nursery, only the youngest has been to school at 3 here, the others started at 4 in the UK. I've not really had personal experience of my own children going at 2 1/2, only watched others. IF we were still in the UK, wouldn't dream of putting the children into a school at 2 1/2, but we're here where it's the norm and for non French / Dutch speakers, school has the advantage of acquiring another language without too much effort.
IF you do ever put your child into school at 2 or 3 , Rachel, just be sure it's what you want and are happy for your child to attend every day, whether part time or full time. It's not a drop-in creche, the teachers do not appreciate children who attend infrequently who take extra time to adjust every time they arrive, make it hard to plan for activities etc. If you are comfortable, your child could start tomorrow, if you are not, I'd enrol and wait till September. Missing classe d'acceuil is not a big deal at all, the only downside is that by not having a place already, it reduces chances of getting a 1ere maternelle place, reduced even more if you wait till 2e maternelle.
Maybe you could look into Farandoline type groups before starting school? Nearest you get to a UK mums and tots group but in French.