I'm surprised that you like hotels near the HBF. In much of Europe, it's the least attractive part of town. Is budget a major part of your decision-making?
I liked Dusseldorf, and it has a huge airport, with good connections. Cologne is actually a more attractive destination for a family vacation (and very walkable), but of course it's an easy daytrip. (I had to skip Wuppertal, which I want to visit, because the commuter train line was under repair.) Note that Duesseldorf is often noted for its beerhall and late-drinking culture, especially in the picturesque old town. That does NOT mean it's not a family-friendly destination, by the way. It's traditional for the hosts to seat you at an already occupied table with up to nine or ten seats. As you may know, most Germans under 55 can speak English. I was there for modern and contemporary art, which it is a center of. It is not an important Rhine riverboat destination, like the Middle Rhine is. I enjoyed the Botanical Garden but did not visit the zoo.
You might try to find the date of the monthly "Fish Market", which is actually a Saturday(??) one-day festival along the river, with sausage, beer, wine, pastry, seafood, and similar stands, right along the riverside. It's good fun, and heavily attended by families. I was disappointed by the thin crowds at the "oldest" (?) restaurant, Brauerei Zum Schiffchen,, but I had a wonderful evening at Brauerei im Füchschen. I was seated with three German chemical executives, and three hard-drinking Chinese staff who were in town for training. I paid for my pork knuckle, but could not pay for my beer and Killepisch shots!
Edit: Clearly it's not on your program, but there are a surprising number of over 100 Euro prix-fixe gourmet places in Duesseldorf! One of the obvious daytrips from Duesseldorf is Aachen (maybe easier from Cologne.) But if you can find a way to get there, even a tour bus, Monschau is special among the dozens of Fachwerke towns. Munster is a little far, but a lovely little city.
I am not trying to get you to change your mind, but you may decide to go on more than one daytrip to Cologne. It's such an attractive and rich destination. Don't overlook some of the less visible destinations, like the Jewish archaeology district (with a sometimes visitable medieval Mikveh), and the extensive underground Roman fort and tunnels. The MAK (Applied Art Museum) in Cologne is especially enjoyable for people who get tired of pictures on the wall. (There is also an underground museum in Duesseldorf, but it's contemporary art on the walls.)