We visited the Black Forest a few years ago and stayed in Staufen at the Gastehaus Kaltenbach. We LOVED it. Our trip was pre-child (our little one is 14 months old now), but I think our toddler would love the "rural" setting (about a ten minute walk from the town center/train station). There's a garden, horses out back, and a friendly farm dog, with plenty of room for kiddos to run around. The breakfast there was incredible and the hospitality was amazing. On our trip, we rented a car, but Staufen looks accessible from Heidelburg (2 changes, about 2.5 hours). The town itself is very charming and has cute and picturesque side streets. I'm not sure whether they offer cribs/portable cots for children to sleep in, but it's worth asking.