swimming lessons

I would have suggested lessons there too, but the pool is actually closed for an "indetermined" period, due to problems with the roof which was replaced 4 years ago following a fire.
http://www.uccle.be/fr/services-communaux/sports
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If you want to travel as far as Uccle, then perhaps David Lloyd club is worth a look. I can't immediately see info about children's swimming lessons there but I'm certain I've found mention of them on their website before. Worth a phone call certainly, but I'd expect them to be quite a bit more expensive than public pools and perhaps more expensive than Darwin Aqua club too.
http://www.davidlloyd.be/index.php