Family trip starting and ending in Cologne

Easy to do flexibly, so why not grab flights now and plan from there. Look at fights in and out of Cologne, Duesseldorf, and Frankfurt, as all are pretty convenient.

Thanks Sam. Have booked flights 23 may to 4 June into cologne as that was the best price, now on to planning the route !

For kids you might want to check out the Cologne chocolate museum.
The Wupperthal monorail might also be interesting for kids. I haven’t been there myself yet, but it speaks to the kid in me that loves trains.
In my view the best castles are Burg Eltz on the Mosel and Marksburg on the Rhine, since they are the only ones that were never destroyed. In Bacharach on the Rhine you can stay in the youth hostel in Burg Stahleck. While the interior looks like any modern hostel, the exterior still looks like a proper medieval Castle and the views of the Rhine are gorgeous.

Burg Rheinfels in St. Goar is a big ruined fortress which is fun to explore. Take a flashlight to explore the tunnels.

The Cologne Ethnographic museum is oriented towards showing children life in the rest of the world, and it's paired with an adjacent medieval art museum located in a lovely deconsecrated church. Your children may be old enough to enjoy the MAK applied arts museum, with mostly three-dimensional decorative arts and historic (and modern) furniture.

Check transportation possibilities to Belgium's Bokrijk outdoor building museum, and adjacent amusement park. I've only been to the outdoor museum.

Cologne and Duesseldorf both have lovely botanical gardens.

One of the biggest theme parks in Europe is Fantasialand, which is in Bruehl a short distance from Cologne. You can get to Bruehl by either the train (faster, less frequent) or U-Bahn (slower but more frequent) and there are shuttle buses direct to the park from both Bruehl railway station and Bruehl Mitte U-Bahn stop.