Fees for a 4 year old, full time (half day weds), self funding
ISB 25k (know the school rarely offers reductions to new students)
International Montessori (Woluwe) 12.5k
BJAB 15.7k
BISB 15k?
BEPS 16k? - 21k
Roots and Wings 10k? - yes the school now takes primaire
BSB 12.5k-17.5k
St Paul's 18.4k (don't know their percentage reduction)
As you see, there is wide variation in fees, if you're paying yourself, I'd be thinking long and hard about costs. Are they really worth it, given you could get exposure to English easily outside school and you'd never be paying these sums of money for extra-curriculars in English.
International Montessori (Woluwe) is very popular, have you checked there is even a place there?
International Montessori is 10km from Chatelain, it's a 60 minute journey on public transport or 30 minutes by car
BJAB is only 4km away, 20 minutes on the 81 tram (so long as the tram doesn't get stuck in traffic) or 15/20 minutes by car - this one is by far the easiest for continuing to the EU area
ISB is 6.5km away, about 30 minutes by 94 tram/short walk and about 15/20 minutes by car
BEPS is 2.5km away, 20 minutes by 94 tram/short walk and 10 minutes by car
Then you have to add on the time taken to go to work. None of these schools are between Chatelain and work.
Is there any chance at all that it's not too late to consider another area to live in? Montgomery/Mérode is very similar to Chatelain but nearer to work, easy to commute to work, local schools which might still have some places!
When moving to Brussels, it is always wise to choose a school first and then choose where to live second. I don't mean to sound mean and nasty, it's however the mistake people make most often and then come to regret and either accept spending a part of their day taking their children across the city to school or they move or they choose a highly inappropriate school and come to regret it.
You could consider a local school a bit of a distance away from your home, but at least on the way to work. You might find places still at Jardin des Ecoliers (Rue de la Croix 37) or Cardinal Mercier (Rue du Nid 8)