If anything, forcing people to speak the language they speak badly at home will make their children's speaking it worse. If a child constantly hears spoken language with a strong accent/incorrect grammar, etc., s/he may pick those up. If the "badly-spoken German" transmits across generations, it might be a sign of too much German being spoken at home: it might be a dialect emerging, perhaps, afrikaans-style. In that case, if you care that second-generation kids speak better German, you should, probably, discourage their parents from speaking German in front of them.
No, that's not quite how it works. Kids are crazy language regularizers. They take whatever they've given and try to make it systematic. That's how creoles are formed; kids hear a pidgin (a simple way to communicate between, say, traders), desperately seek any patterns they can, and then turn it into a fully-fledged language of its own. In a situation where somehow the parents were the only input to the kids, you'd expect the kids to regularize whatever the parents were doing. It might be different from the standard language (I mean, Tok Pisin definitely isn't English, and as you say Afrikaans is not Dutch), but it wouldn't be "bad" like second language speakers are sometimes "bad" (unsystematically irregular).
However, that assumes that parents are the main arbiter of how children speak, which is also mistaken. Although parents in Western societies are often the first people who model how to use languages for children, that doesn't mean they determine everything. This can probably be seen most easily in exceptional circumstances. There are plenty of cultures around the world where parents hardly talk to children, yet the children learn language just fine... because they have peers and siblings speaking to them instead. Another exceptional circumstance was the child-rearing habits of the English upper class. Kids were basically bundled off unceremoniously into the hands of successive lower-class caregivers until they reached an age where they could be shipped off to boarding school. Parents were encouraged to ignore their children, so the kids' primary caregivers were these lower-class folks, who presumably spoke as such. And who did they talk like? Well... their parents. Because you know who else sounded like their parents? Their peers.
This is also true in terms of immigration. My child language acquisition professor in undergrad was a Brazilian by birth. Her English was good, but she was clearly accented, and some syntactic things in English were challenging for her; as she put it, her input to her daughter "was a deesaster". Does her daughter also speak accented English? No, of course not. Nor does she speak exactly like her dad, who's a native speaker of English, but Canadian. She talks like her Michigander friends at school. Cultural spread isn't parent-to-child. In fact, we'd probably find it kind of alarming if it was. Imagine if all of us here had the computer skills and attitudes towards technology of our parents. Would any of us be posting here? Cultural spread is peer-to-peer; it just happens that there's usually a correlation between the attitudes of one's parents and one's peers.
As such, to the extent that non-Germans in Bavaria speak "worse" German, it's likely just a consequence of the segregation of non-Germans from Germans. If the immigrant population were located such that the kids of immigrants had "native" German peers, it's likely that they'd become Germans in thought, deed, and speech. Regardless of what happens at home, once they got out into the world and made some new friends at kindergarten, they'd quickly realize the seemly thing to do would be to speak German just like everyone around them. Undoubtedly, this tendency towards clustering around the in-group isn't just the result of the attitudes of native Germans; I'm sure self-segregation plays a role, too. But policies like this ridiculous farce of a proposal certainly do no favors for the ideas that they're supposedly promoting. The best way to ensure a new group doesn't talk like you is to keep driving home the idea that they're not like you and not one of you.
