I don't know when you want to travel, but next to campsite "Goldene Meile" at Remagen there is an Open Air Pool with slide. Nothing fancy, but my kids (9 and 5) like it very much.
We stayed there a few time and always enjoyed it. It's about 40 km to Koblenz.
At Bad Hönningen you can find a campsite next to a Thermal Bath (Wellness, SPA etc.). Haven't been there yet, but heard good things about that place. However no "fun" bath for kids.
Suggest you check before booking, lots of pools have been closed early as the weather has been so bad.
Heard it on the News while we where there in August.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.