Where you live depends on how much you like commuting and your budget. Buying here has very high costs, not recommended unless you know you'll be here at least 5 years or you don't mind losing 10-20% of house value in costs.
Options for school are private English or public French or Dutch.
The cheapest private English school with a track record longer than 2 years old is International School of Flanders ISF, for those funding themselves. There is also Bogaerts but little more than some classrooms rented from SSB and with another "school" opening on the same site, same way of renting classroom form SSB, called European School Argenteuil (no secondary years), I wonder what will happen there and would not be confident at all that European school Argenteuil or Bogaerts school would still be running in 2 years. ISF is a long standing school but has only done 16-18 years for about 4 years, the choice of A levels in limited. It is however only 18k per year for self funders.
www.wis.be
www.bischool.com/en/index
Then you have a huge step up to the 3 large private schools which cover secondary school age, you will need 30k per year as a self funder minimum. ISB, St John's are both effectively American and BSB is British curriculum.
If you are brave enough to go French, I'd recommend Le Verseau. It is NOT a private school and it is NOT an international school (despite the name). It has, like many Belgian schools, a large number of nationalities and significantly around half of the secondary students are mother tongue English. The head is British. It also charges fees - public schools are allowed to in Belgium - but only around 5k per year. 4-6 hours per week mother tongue English, rest in French. This would be the public school best matched to English speaking children with limited French to go cold turkey into French. You can also hold your children back a year to concentrate on language acquisition and take off the academic pressure. You can also do A levels and GCSEs, not at Le Verseau, but supported by Le Verseau staff as an external candidate at BSB for 100 euro per exam. It's quite a small school, something like only 220 in 6 secondary years. It's not far from Waterloo to Le Verseau and there is public bus which goes from Wavre station connecting with train and other buses and goes to the school once a day.
www.eiverseau.be/secondaire/fr/index.html
Here is a map of the private schools.
batchgeo.com/map/0777fe5f939fe09f11d58a91ceee1066
Then you can basically