I did explain that we spend the summers in Sweden. Dd who is an unbelievably social person goes out and makes friends locally and then keeps in touch with them during the year. She has a much stronger regional accent than I ever did and her language skills seem very up-to-date as far as I can judge. She has also got Swedish penfriends and has recently joined an internet forum.
But I fail to see how OPOL is relevant to this particular aspect of language learning- modern slang isn't something she would be learning from her 45yo Mum anyway. It's what you learn from your friends. As you point out, it was an OPOL lady who said her kids sounded middle-aged when they spoke Swedish- so OPOL clearly hadn't made that much difference- one can see why.
Anyway, slang is something you can pick up fairly quickly: if my dd did not know the latest slang but had a good general grasp of the language, I would think that was quite good enough to be getting on with. Besides, slang varies regionally- if I had moved from Gothenburg to Stockholm as a child I would have sounded middle-aged. Didn't mean I wasn't a native speaker.
Nor do I believe that dd's emotional attachment to Swedish is going to be that much fainter because I occasionally speak a bit of English. She has always considered herself to be half of each, and so does ds.
Her identity is being bilingual and her strongest role model is her bilingual Mum. Copying me would not leave her monolingual, quite the contrary. We are a bilingual family and being bilingual is the norm as far as she is concerned: it's what everybody in her family does. (though ds, bless him, went through a phase of refusing to speak English when he was about 3- his playschool were very good about it ).
Admittedly, dd has not yet reached her teens (just about to turn 12), but she has been through some pretty traumatic and stressful times (due to disability and misdiagnosis) and has been fairly rebellious with it. I have taken just about every kind of crap you can from a pre-adolescent daughter, but it has not involved language. Perhaps because she doesn't think of bilingualism as something we are trying to impose on her, it's part of her own identity- she can't imagine not having that.
We also have several other bilinguals in the extended family (one aunt is Swedish/Chinese, one uncle works in Norway, both grandparents speak all the major European languages). To her, as it was to me, it's a normal thing for an adult to know several languages.
And she is emotionally attached to me as a Swedish speaker too. In fact, primarily as a Swedish speaker, seeing that I do speak more Swedish than English. If I occasionally coach her in her drama exercises or read her some Shakespeare, this is not going to negate all the hours we spend and have spent sharing jokes/telling stories/arguing in Swedish.
(It is admittedly a bit of a special situation in our family, as I am also better qualified than dh to teach English literacy- though a native speaker, he finds it difficult to express himself in writing, has read relatively little and is no good at spelling. And can't recite poetry without provoking howls of anguish from his audience, bless him! It's about as good as listening to a Vogon. As dd has spent over a third of the last 4 years absent from school for health reasons, this has become rather important. She would not be top in literacy if Mum hadn't taken responsibility for that side of things too. But I have yet to find that any harm has come of it)