Depending on exactly how you define a "14 day" vacation (how many full days will you really have? do not count your arrival or departure days), I would advise caution not to try and include too many locations. For most people, a "14 day" vacation means you get around 11 or 12 days in Europe (I'm assuming you will have a long flight to get there and to return, so your arrival and departure days are not really usable).
Also be careful about how much your family members can do in a day - in a group, you only move as fast as the slowest member of your group. Also, in mid-winter when you are planning to go, days are short and the weather is often wet, which can limit the things you will do (on the other hand, the crowds are thinner).
Give all of that, with just 11 or 12 days in mid-winter and a family along, I'd suggest you limit yourselves to 3 or maybe 4 locations maximum. Of course, although Belgium and Netherlands are relatively small countries, France is a large place - so you're going to need to be selective.
I'd consider something like the following:
Fly in to Paris, fly home from Amsterdam (flights that depart from Amsterdam and usually less expensive than those that depart from Paris).
Allocate roughly 3 days per "location" (you will have to slice the time based on how much you actually have).
Travel by train between cities.
- Paris
- Bruges/Brussels
- Amsterdam
Hope that helps. Good luck.