Farm stay: We stayed 3 nights at a great little farm in the Black Forest (Steinach) operated by the Schöner family a few years ago. The upstairs apartment has a nice view of the farm and surrounding countryside from its back deck. Mr. Schöner speaks English and is very friendly - you should browse the photos below at his German-language webpage; the availability calendar there shows vacancy throughout June:
Steinach has a grocery store and bakery and a restaurant or two as well. We walked about 5-10 minutes to the Steinach train station from the farm to do our outings - Steinach is a stop near Haslach on the Black Forest Railway - and it was very easy to get to all the other Black Forest Railway towns from there - and free with our KONUS card. If you wish to book I have 2 email addresses - you can book directly with him. Ask to receive an "offer" first - he'll give you the full price for your stay (possibly including a short-stay fee if you only want 2 nights) but it will still be quite reasonable, I think. I have two email addresses:
He has parking space for cars too of course.
I've done a lot of driving in Germany and of course you could rent a car - lots of people do.
The actual transit time for the train journey I suggested is 6 hours (that excludes the 3.5-hour break in Lindau; you could arrive earlier in Füssen if you wish by making your stopover in Lindau shorter, of course - just leave on an earlier train from Lindau station (which your day pass allows.) Viamichelin shows a driving time of 4 hours between Hausach and Füssen; that of course does not include restroom stops, fuel stops, traffic delays, detours of your own or someone else's doing, or car pick-up time at the rental outlet - none of which are needed with the train ride. So while you might save SOME time, it likely will NOT be 2 hours.
I picked Lindau as a stopover as it's the most scenic and interesting place I can think of to stop over en route and near the middle of your journey. If you wish, you could stop over by train elsewhere as well - Radolfzell is a nice town, also has lockers for bags, and is right on your route.
I pretty much refuse to drive in Germany unless it's completely necessary. Driving is a lot of work, responsibility, and expense. Riding the train is simple and involves no contracts, no insurance, and virtually no risk of an accident or injury. You get comfortable and enjoy the scenery, people-watch, read, break out a picnic, play cards, get up and stretch your legs, do whatever you like. The roads are are lined with radar speed traps all over (lots of us have gotten tickets in the mail back home afterward) and there are lots of complaints online about shady German rental outlets that trump up repair costs for minor scratches and dings that the driver was not aware of (yes, they send those bills too.) And of course you will already have been driving all over France. The train in Germany will be a nice change of pace and probably a real diversion for the kids, if not the adults as well.
Map of all Bavarian train lines: Lindau is at the bottom left: