Check the Benelux forum, too -- much more on Belgium, there. Offhand, I'd say that, depending on the ages of the kids, they might be interested to see the brewing works and barrel rooms at Cantillon. If you're going to force them to spend time in a pub (we won't call child protection services), Toone and Poechenellekelder have puppets and suchlike bric-a-brac that might amuse a youngster for the duration of 33 or 75 centiliters. At Toone, the actual puppet theater performances could be hit or miss -- hey, puppets in action, but also in French (or rather Bruxellois dialect, it seems). There are comic books at Moeder Lambic but, not the type for kiddies, of the more serious (such as they are) variety, so don't let anyone gull you into that. You can probably get a beer up in the Atomium, and might be able to walk around with one at Mini Europe, but I can't imagine it would be the most interesting beer opportunities -- maybe a Trappist but I haven't done that, myself. After saying all this, I might as well admit that we're only a small step away from suggesting that you brown-bag it on the city bus tour. Let's not go that far. Further afield from Brussels, I might be able think of beer gardens with playgrounds and the like. But in Brussels, well, that's already pretty good for two usually mutually exclusive activities -- drinking beer and entertaining kids.
I bring my daughter, who is 3, with me almost everywhere. No one has ever complained and she has been in more breweries than most adults. It is a much more open society here than in the States.
Good to know. My wife and I were in Brussels back in 2011 before we had our (now 3 years old) daughter, but we're heading back this June with her and my beer loving parents. I figured she'd be welcome most places, but didn't want to assume. She's really used to going to breweries (Seattle is very welcoming everywhere except for bars), so that's not a concern.
Certainly not standard equipment that I would expect everywhere. There's probably less taboo at the idea of a kid in a brewery than a kid in a restaurant (half joking). Hopefully some others will have specifics to share for where it worked, or didn't?