"I will fly into Frankfurt...and plan to stay near as our first base..."
Since you have "10-14" days I assume your flights are not scheduled. If Garmisch and Salzburg are in the mix, then you can save yourselves some ground travel time by flying home out of Munich.
"I would love to find out places people would recommend basing out of, places to stay in those areas and things that you would strongly recommend doing."
It would help to know your family's interests as well as the children's ages and the mobility levels of your parents.
Rüdesheim in the Middle Rhine Valley comes to mind as a good place for oldsters, one with several diversions. It's roughly an hour from FRA airport.
River cruises through the Rhine Valley past castles, vineyards and old-world towns
Gondola ride over the vine-clad hillsides to lookout
The Drosselgasse in R'heim's old town (cafes, shops)
Marksburg Castle in the riverfront town of Braubach lies north of R'heim on the same side of the river. It's the finest example of an intact medieval Rhine castle and easy to reach from R'heim.
The Loreley-Bob summer bobsled run is in between R'heim and Braubach - could be fun for kids. And just across the river from there is the town of St. Goar, home of Rheinfels Castle. Use the car/passenger ferry there to cross.
Rüdesheim has its own ferry crossing too. You could take a car outing over to the spa town of Bad Sobernheim to check out the open-air museum, where you can catch a glimpse of local life - something of interest to those of German ancestry, I would imagine.
A stay of 3+ nights would be good for this area. Your first day is likely to be messy (jet-lag, general grogginess) so don't plan much that day.
DRIVING: I don't do it in Germany anymore. Good driving primer here from The German Way site. I'm sure you know that the roads tend to be in fine condition but also tend to be narrower. Parking too. Radar controls are everywhere and intolerant of those who break the speed limit. Complaints about German rental agencies are ubiquitous on the travel forums where I tend to hang out; biggest complaints seem to involve very expensive repair bills in your home mailbox - some agencies are very skilled at finding scratches and dings that you didn't actually make. Inspect the car carefully and take 100 photos when you pick up and when you drop off?? The Autobahn's high speeds can be stunningly stressful. Driving (in unfamiliar territory) after an international red-eye flight is generally not a good idea; I would take the train to R'heim (or other first stop town) from FRA airport - then pick up the van later (possibly in nearby Wiesbaden.)
