What I've found is that if you don't actively speak your native language and your day to day language is different enough from your native language sound-wise, you lose the ability to voice some sequences of syllables smoothly, because your mouth and tongue are not used to it anymore.
Indeed. I should have emphasized above my second point better (instead of the proud parent digression I wrote) - my son is the only one I speak Hungarian with every day (and vice versa). I’m certain that without this I would’ve lost a very big part of my Hungarian language skills over the past 6+ years.
I'm in a similar situation. Living in Germany with a German wife, German neighbors, German speaking friends, my children are the only people I regularly speak Hungarian to. My 6yo daughter speaks well enough when she's around her grandparents for a few days, but with my 3yo son the situation is different: he was born into a family with a German speaking sister, so he barely speaks Hungarian.
The fact that I'm the father, who works more and spends less time with the kids than my wife doesn't help either. I guess it's called mother tongue for a reason.
I have found music to be the key, they absolutely love for example the Sebő Együttes, which is indeed absolutely fabulous music. Their favourite is this: https://youtu.be/H8SY89dSDa8
I only speak German with my children (living in Sweden) and one thing I encountered is that if I don't pay attention I automatically start speaking in German to all children. It has gotten me some weird looks from some of my kids friends over time. Do you notice the same thing?