Conrad - there are soooo many chocolate shops in Brussels - it was hilarious! We went to the main square and wandered around the surrounding streets (we had a few hours to visit on our way from Ghent to Paris) and if there were 20 stores on a street, I swear 17 of them were choc shops - and every one offering a sample...oh my!
When we visited Bruges, we did a canal boat ride (fun)...we also did the Historium - just...don't. Skip it, unless you want to watch what basically was a short movie about a guy, a painter, a bird and a girl...can't believe we spent the money on that. After that debacle, we passed by somewhere offering a 'chocolate walk' and kicked ourselves for not doing that instead.
Airbnb or hotel would be up to you. I tend to airbnb now to save money, but my husband and I don't mind staying in a room only with the owner present (as opposed to whole apartment). If you thought you may want somewhere to be able to prepare your own meals, have breakfast items or snacks on hand, an apmt may be better - especially with kids - you may want to have separate rooms for privacy. You can look at VRBO as well as airbnb and some other apartment rental sites as well.
We like to use public transport more than having a car, but there are times when a car may be better. I think you'll have to look at where you want to visit and the ease of getting there by train/bus. We debated having a car in the NL when we left Amsterdam, but with only 3 nights, we figured what we wanted to see was doable using trains (of course, we were there over Easter weekend and they were doing rail work near Dordrecht. We had some delays and had to use buses to get to other towns so we could catch a train to where we wanted to go). If you want to explore smaller villages, or visit some of the war sites, a car may be the better option. We have only rented in France - once when we were exploring the south of France and wanted to see a lot of less accessible places. It can help and hinder finding cheaper places to stay - help because you can stay a little outside a city where it could be cheaper, but hinder because you need to be sure there is somewhere to park, preferably free and onsite. But being at the whim of train schedules can be a PITA...we stayed in Ypres for the Last Post. There was one train an hour back to Ghent, and of course, LP being over at about 8:20, we had to wait until 9:15 for the train back to Ghent - nothing was open, it was cold...we couldn't even wait in the station as it was locked up - there was a tiny little French fry place where we hung out for 40 min. And an hour back to Ghent, and by the time we got back to our room, it was after 11pm.
In the NL, we stayed in Dordrecht, which was nice. We visited Gouda and did a nice walking tour via a book we bought at the tourist info/cheese house. Also visited Kinderdijk to see the windmills, which was great. Also stayed in Amsterdam 5 nights and did some daytrips from there.
I could see 5-6 nights in Belgium giving you a good overview. I can see going back some year for another 4 nights or so when we finally decide to see more of Germany, and I'd love to go back to the NL for another week and really see more of it (most likely would get a car to see some off the beaten path places, but for a first time visit, public trans would be fine).
One thing you could look into - when we were in Ghent, we bought a 10 ride pass for the trams/local transport in the city. Those were also good in Bruges as the same company ran the trams or bus there (we just walked in Bruges tho). Now, not sure how widespread they are, but that may be something to research that you may get a pass that would be good in more than one place for the local transport.