Gengenbach is small, Freiburg much larger.
I would consider consolidating your stays. It sounds like you may have 5 or more days for the area... From Gengenbach you could easily day trip to Baden-Baden, to Freiburg, to Strasbourg, or to the towns along the scenic Black Forest Railway line (also accessible by car if that's how you're traveling.) You could rent an apartment in/near Gengenbach with 5+ days - nice to have that extra space and a kitchen - and maybe even an outdoors area of your own - when you have small kids.
The Vogtsbauernhof open-air museum in Gutach is a particularly interesting destination. Not far at all from Gengenbach.
http://en.vogtsbauernhof.de/ (toboggan run next door... http://www.blackforest-tourism.com/Media/Attraktionen/Toboggan-run-Gutach )
BTW, the BF Railway and all other railways around the Black Forest can be traveled for free if you stay in Gengenbach, one of the towns which offers the KONUS card (guest card for rail & bus and for area discounts.) The card would cover day trips to Baden-Baden and Freiburg and many other towns as well. You cannot get this card if you stay in Baden-Baden (which actually is mostly about casinos, spa treatments and shopping) or in Freiburg.
KONUS info: http://www.dreisamtal.de/en/service/konus.php?lang=en
BF Railway: http://www.black-forest-travel.com/places-of-interest/black-forest-railway.html