In Paris - get a Batobus ticket and do a cruise of the Seine up to the Eiffel Tower and back. You can get on and off at various places. The fountains at the Louvre and a stroll through the Tuileries is lovely. Les Jardins du Luxembourg have various carousels, a fantastic playground and puppet shows.
In Brussels, you have to see the Grand Place and the Mannequin Pis. Kids eat free in Chez Leon on the Rue de Boucher. The Royal Palace is normally open to visitors in the summer, and there is a big park with playground in front of it. There are various good museums - Natural History, Musical Instruments good for children. You could spend a day/afternoon at Heysel - get metro/tram from the city centre. There you have the Atomium (more impressive from the outside) -and Brupark, where there are lots of restaurants, Mini-Europe (model village), a multiplex cinema, Oceade water park, carousel etc.
We have just booked a summer trip to Europe and will have one day in Paris and two in Brussels. Any tips on places we must see given the limited time or just general advice on travel with young ones? I'm after something to interest the 4yr old as well as Dh and myself, that isn't too expensive and won't involve lots of walking. We will be tired as the rest of the holiday will be at Disneyland. Thanks in advance.