We are leaving the North of the Rhine Valley in July, and thinking of doing a 'loop' to get to Hamburg. We are planning for this loop to take about 2 weeks, with 4 adults and 3 children. Our proposed itinerary is Trier, Brussels maybe, Ghent, Bruges, maybe Antwerp, Amsterdam. We are extremely flexible and Budget travellers (3 star and Youth Hostels) and always open to innovative ideas. I am hoping the Rick Steves community may be able to help with suggestions.
Trier is primarily of interest if you are into either Ancient Rome or Karl Marx. The most direct rail route from Trier to Belgium would be via Luxembourg. It's quite a scenic trip but nowadays doesn't have particularly fast services. It may be faster to go to Cologne and then take a Thalys or ICE to Brussels.
Belgium is sufficiently small that you can stay in any one of the cities you mention and take day trips to the others. There is no advance-purchase discount for Belgian domestic rail journeys so you could do them on the spur of the moment. The Belgian railways also offer a cheap 10-journey pass which can be shared between more than one person.
Many people here dislike Brussels, but I like it, as it has some very interesting art nouveau buildings and good art museums. However, it isn't very good for a day trip, as the interesting sites are spread out around the city. Bruges is very much tourist-focussed, while Ghent and Antwerp are much more working cities. They all have some interesting medieval and early-modern sights.
You will get significant rail ticket discounts if you book your journey from wherever you stay in Belgium to Amsterdam in advance. There are a couple of different operator and website options, so have a look at http://www.seat61.com/international-trains/trains-from-Brussels.htm#Brussels-Amsterdam.
Trier and maybe even Brugge might bore the kids. They might get a kick out of Het Gravensteen in Gent, though.
You could also consider a side-trip to the beaches in Belgium. I particularly like De Haan. It's a beautiful little resort town and very family friendly.
Brussels has a little more stuff that might interest the kids. The Musical Instrument Museum usually goes over well. The Comic Arts Musuem... usually less so, unless they already have an in-depth familiarity with characters like Tin-Tin, Suske and Wiske, Lucky Luke, De Kiekeboes, etc. Unfortunately, I think MiniEurope has closed because they're redeveloping the property into a luxury mall, but it may have moved to another location. This is/was a wonderful little interactive park with models of well-known European landmarks. A great tool to teach kids about the EU.