I'll try as no one else has.
Day in Kõnigswinter - Sea Life plus trip up Drachenfels
Zoo in Cologne or trip to Chocolate museum . The Shokoexpress goes from outside the Cathedral.
Think there is an amusement park thing at Cochem with cable cars.
Boat trip down Rhine on the Moby Dick with trip to Eis Cafe where you stop.
Eifel Park
Eifel Zoo - great little train through the reindeer park where the reindeer come right up so you feed them.
Selection of outdoor pools around - google 'Freibad' or there are a fair few lakes dotted around where you can swim and they have cafe's eg. One of the lakes of the Dauner Maare. Some have a good shallow bit for little ones to play on and slides.
Various crazy golf places, playgrounds, cafes, riding, cycle paths etc.
Those are all spread out over a big areas some accessible by public transport and others you would need a car.
I think maybe one of your best options would be to look at Landal Green Parks or Centre Parks, use one of their sites as will have a fair bit to do on the site then hire a car and do day trips out, depending on exactly where you need to be for the family party. We had a fabulous farm holiday but think it would be hard potentially unless you speak a bit of German.