CV, bruges with 3 dcs those ages will be a breeze.... all the things you want to see are within very small area so very easy to get around and not too exhausting for feeble children
things we did that were good:
boat trip on canals (30 mins) v nice
climb to top of bell tower in main square (366 very steep steps, nearly killed me)
eat large quantities of mussels and frites (available pretty much everywhere)
drag children to two small but perfectly formed art museums (Groening museum and memling) to see beautiful northern renaissance paintings... the first has a fab bosch depicting the day of judgement and is gruesome enough to appeal to small people
play count the chocolate shops... ds2 took a photo of each one he saw and had 43 after 2 days
eat waffles and icecream
visit "choco-story"... the choc museum and the friet museum "from potato to chip" for a laugh
go for a walk away from the main tourist area where there is lovely peace and quiet and admire the beautiful architecture... go to minneswater (a park) which is a green oasis in otherwise very built up city
find a flea market (near canals) and browse...
does that sound enough to keep you occupied?!... or you can always get on a train to ghent....
hope you have fun!