They do have those. If anyone wants to be child free they can choose to dine there instead of a restaurant that markets directly to children via free toys (a problem for a whole different thread).
There are also restaurants that allow children but the patrons are spread further apart so the chance of an interaction with a stranger, at least while eating, are much lower.
If you go to Biergarden at Epcot you will be sat with other families, if you don't want to do that don't go there. Simple.
Not any where I live. I haven't been to Biergardgen for that reason.