How to teach heritage speakers

I hope this is the right subreddit (does teaching count as applied linguistics?).

I am a German language tutor to two trilingual kids. The dominant language and environment language is Norwegian. Their parents speak German and Dutch, respectively, and they sometimes see their extended families and speak those languages. The kids are 8 and 10 yo.

They both have good speech flow in German and an acceptable age-appropriate vocab, with some lexical gaps that are normal for trilingual kids. The problem is, they didn't organically pick up case at all? They mix up all the inflected forms for case, even though they do master other inflected forms such as plural allomorphy. Maybe it's because Dutch and Norwegian have similar complexity in plural but no case? Don't know.

Anyway, their German language parent noticed that they mix up case in pronouns and asked me to practice it with them, and I just don't know how, because this heritage language situation is so unusual.

There are tried and tested methods teachers use with actual native speakers. Native speakers sometimes mix up some forms that are inflected for case, mostly due to phonological erosion (einen => schwa elision => einn => ein), but all native speakers will make some distinctions for case, so the trick is teaching them to substitute one word where they don't make a case distinction with another word where they do make a case distinction, and have them derive the case from there. The usual method is to have kids ask for whatever constituent they need to determine and they will usually pick the correct inflected form in the interrogative (wer vs wen vs wem). If the kids I tutor had access to this case distinction in interrogatives, I could teach them that whenever you ask a question with wem, then the other forms must change accordingly, etc. But they don't distinguish any interrogatives by case. This is the problem that the German-language parent hasn't realized, that it's not just a few pronouns, but that it's deeper and more systematic, because the kids' internal language doesn't have any case at all.

Of course, there are also methods to teach this to L2 speakers, but usually they use way more explicit systematization and they are only targeted at older learners because generally 8-year-olds can't do this level of abstract systematization yet. Everything I read about L2 teaching for kids says that you shouldn't focus on formal correctness at all, and that developing speech flow and vocab should be prioritized, so my approach so far with those kids has been ignoring those case problems and just giving them a lot of prompts to encourage them to talk, but now their parent has asked me to fix those pronouns. Also, typical L2 speakers have zero speech flow and vocab, so there are a lot of low-hanging fruits for teaching L2 children of what you can do with them even if you don't focus on inflection. But with these heritage speakers it's different, because they already have a good speech flow and vocab.

So, the TL;DR is, they are too much like native speakers to apply the methods made for L2 speakers, and too much like L2 speakers to apply the methods made for native speakers, and I don't know what to do. Does anyone have any advice?