Brussels Museums are closed on Mondays (with a few exceptions). On Wednesday afternoons schools are closed and this can mean that some places are inundated with kids, but then they will be on holiday anyway in July, which is the Belgian holiday month. As far as I know, as long as children are accompanied by adults, they can go into a bar.
Nearby Brussels you'll find Brussels Mini Europe wonderful handmade figures from every city in Europe. (prices € 12,90 for adult; € 9,70 for children). To get there take Metro line 1A direction Roi Baudouin-Koning Boudewijn and get off at Heysel-Heizel. It’s very nice for kids. Mini Europe Belgium has buildings from all over Europe reproduced in 1/25 scale. If you want to see a miniature of the Eiffel Tower or the Vatican, you have to go to Brussels Mini Europe. I think an hour would be a lot of time to spend there. But if it's just you and your partner, or friends, save your time for the real city attractions like Manneken Pis.
I think a visit towards Mini Europe and the Oceade Heizel is a nice way of spending time with your kids. It`s a large subtropical swimming pool with 11 water slides, open air and covered zones. Prices Mini-Europe + Oceade for kids €17.90, and € 23.90 for adults. (combi ticket cheaper) Oceade is part of a larger complex called Bruparck. There are several family attractions, lots of restaurants, a movie theater complex near the entrance to Mini-Europe.
The Atomium is also very close by and recently refurbished. Besides the Bruxelles Atomium there is a ferris wheel that takes you almost as high up as the Atomium Brussels. The view from inside is great.
Aquarium Public de Bruxelles (prices adults €8, and € 5 for kids) has fish and other sea creatures from around the world. Brussels Aquarium is a trip in aquatic environments of all continents. It is an open window on the nature on 1200 m2 in the heart of Brussels.
Neptunium is a huge water park which is also an exciting family destination.
Walibi is an amusement park just outside Brussels with over 50 rides, including the only wooden roller-coaster in Belgium. It is for a day full of exciting moments for kids. whether it's in the water, on the ground or in the air
I would recommend you take kids to Technopolis in Mechelen, exit 10 off the E19. It is a terrific indoor play and learn center that has many interactive science projects and toys. It is appealing for kids there are lots of things to climb on and play with.
There is Autoworld at the Cinquantenaire if they like cars. which is a huge collection of cars dating back to the 1900's. Never seen my kids so excited. This is situated in the Parc du Cinquantenaire, where there is also a children's playground.
In Musee des Enfants (The Children's Museum) your kids can learn about themselves and discovered their talents, feelings and senses, they can paint, bake, play, create.
The only museum in Brussels about chocolate is Musee du Cacao et du Chocolat. You'll find this just off one corner of the Grande Place at 9/11 Rue de la Tête d'Or, a little street just to the right of the Tourist Information Office. Prices : for adults €5.50, and free for kids. It is very small, but it is nice. During your visit your kids will enjoy learning everything about the origins, the discovery and the arrival of cocoa in Europe. In this chocolate facktory they will enjoy with a biscuit dipped in lovely gooey fresh belgian chocolate.
The parks in Brussels are good, with playgrounds and lots of room to run about - Cinquentaire - with it's car exhibition, Parc de Bruxelles and the Bois de la Cambre the south park Brussels is gorgeous, acres of lakes and woods and lawns - good for walks and picnics, but it's also possible to hire bikes and is car free on Sundays.
If it rains I can highly recommend the Musical Instrument Museum. Everyone knows that music is a universal language. it is not far from the Central Station; you can also can get there by metro to Parc and then tram, e.g. 94 Direction Hermann Debroux, to Place Royale. Not only is it housed in a lovely art nouveau building, with a café on the top floor, but you get headphones to tune into the music played by the instruments as you pass their display cases. There are some interactive displays in the basement. My family loved hearing all the different sounds and comparing the historic instruments. If you are caught in rain near the Grand' Place, head for the Galeries St. Hubert, which is of course under cover and full of shops and cafés.
My kids and theirs friends are already planning a date of Brussels trip.