A family of 4 adults in Germany would do well to hunt down a "Ferienwohnung" (vacation apartment) or "Ferienhaus" (vacation home) in the towns they've targeted. Most Americans are familiar with the "Airbnb" company, but if you only use that website, you will miss out on many options that might suit you. The local and regional tourist office websites will give you the widest selection.
For this reason, sharing the names of the towns/cities/regions you might visit would help us help you more effectively. There are travelers on this forum who are probably familiar with the places you want to go and with the local offerings and the local websites that can help you.
Going to the Black Forest? There are thousands of rentals there:
https://www.schwarzwald-tourismus.info/planen-buchen/uebernachten (click on "translate" for English and search "apartments and holiday homes", "farms", or "accommodation for families", then enter data in the "accommodation search" box on the left.
Individual towns/cities often have their own individual listings. Many towns have their own tourism websites. Others have a page with the local tourist office. It's not unusual at all for a town to have both English and German-language pages.
"Unterkunft", "übernachten" and "gastgeber" are good search terms to find accommodations if you locate a town website that has listings just in German. "www(townname)de" is a good place to start. A search goes something like this... Say you want to find a rental in Boppard... try...
www.boppard.de
Click on "touristinfo" and you get to the tourism page in German:
https://www.boppard-tourismus.de/
Click on the British flag for English-language tourism pages and start looking around in English.
Osterspai is one of my favorite little spots on the Rhine River. Just 1,200 residents. Googling Osterspai and Gastgeber brought up this page of listings - 13 options, most of which could in fact house you if available.
https://osterspai.welterbe-mittelrheintal.de/urlaub-und-uebernachten/gastgeber
I know, you weren't planning to go there. And you probably will struggle getting through the German to see whether you even want to google the establishments any further. But the point here is that people on this forum can help you get to the right pages or point you in the right direction if they know what you're after.
A stay of 3-4 nights or longer will most often yield discounted rental fees (or eliminate "cleaning" fees) so it's often wise to pick a town that can serve as a travel base for other nearby towns that interest you.