There are loads of kid-friendly activities in Brussels.
One fun attraction for the whole family is the Belgian Comic Strip Center. For kids with a sweet tooth, the Museum of Cocoa and Chocolate is bound to delight, and young kids will enjoy the variety of activities at Stardust Park.
There are also other indoor playgrounds like Stardust Park which are also good options. Otherwise, one of the many parks and green spaces in Brussels could be a good place to hold a birthday celebration, for example a picnic!
How old are your kids? Wishing them a happy birthday!