Summer stage for 5 year old

A frequently asked question. There are masses of stages available in French and Dutch throughout Belgium, for Brussels region in French, you can look here - several hundred to choose from, too many to list here.
http://www.bruxellesenvacances.be/
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English stages are available at the 3 large international schools in the greater Brussels area and often at International Montessori too. Look on the school's websites. The BSB ones are usually the cheapest, but expect 2 or 3 times the price of comparable French language stages.
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A tip, cheapest stages in Auderghem are going to be the ADEPS ones at the ADEPS sports centre in Auderghem.