You pay fees to Singelijn because you do! They have a bit more freedom in their curriculum, for the privilege they get funded less, parents make up the shortfall and pay the teachers' salaries, you're not paying for any extra services. For 3 children I think you'll pay less than 4k per year. Schools lunches are more expensive, garderie fees are 2 to 4 times more expensive, no lunch time activities like most other schools, all after school and 2 times the cost of other schools. Fees keep poor children out too, some parents see this as an advantage. It's a great school though. But only you can know if you want to pay for it, it's just a local school like others, except for the money side. Go and discuss it with the head.
It's up to you. You are surrounded by 4 excellent Catholic schools much of the rest of Brussels would be envious about : du Bonheur, Divin Sauveur, l'Angélus and St Henri, they all feed naturally into DDM and SCdL secondaires or maybe even St Michel. Depending on what end of your street you are, I'd choose any of these schools.
Don't forget you are unlikely to get permission this year to move your 7 year old, unless you move fast with getting the form filled, moved the other 2 first, try and use that as a justification for moving the 7 year old, probably won't work but worth a try.