stages for French for teens

I suppose it depends on your budget. Kiddyclasses is the biggest and possibly the cheapest provider of French courses for non francophones, held at ICMEC in WSP, but class sizes can be big. You could opt for more expensive CLL with smaller classes. They hold them at their building near Kraainem metro and in the Summer at ICHEC in WSP. CLL do stages also in conjunction with Mikado Club at lycee Mater Dei for example in WSP.
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But if you think a few weeks in French language stages will make a child significantly better at a language, bear in mind it takes several hours a day over several months for a small child to become fluent in another language and it only works if there is sufficient quantity, consistency and a need to speak that language - that's why bilingual schools in Brussels too often fail to produce children fluent in French and English. You could consider instead sending children to practice sport or music or arts in French over a long period of time and for a number of hours a week.