I can't recommend a farm stay close to the lake but these might help, here and here
If using Mona's link, look for accommodations in the Westallgäu, the region closest to the lake. The Allgäu is a pretty area in its own right.
We once stayed in the lower part of Meersburg for a short time. On certain days you can catch a brunch or evening dinner cruise, if that appeals to you. Hagnau, the next village over, is quieter and has some winery stays up in the hills.
There's a monkey forest not far from Meersburg: https://www.affenberg-salem.de/
A few more things to do with children: https://www.bodensee.eu/en/what-to-do/lake-constance-highlights/families If you can afford to splash out a bit (I can't), there are infrequent Zeppelin flights from Friedrichshafen.
Meersburg, Lindau and Uberlingen are all worth a visit, as are Salem Monastery and Palace. I really liked my visit to Insel Mainau. Bodensee tourism: https://www.bodensee.eu/en