yes i've found quite a bit online now. I'm not worried about it, just want to make sure i'm ready - i think its a case of just more admin but otherwise it's ok. You can use ID until Dec 2020 after which they'll need passports. Then a letter saying you're employing them as and when they cross the boarder post October 31. I'm not sure whether the points system makes a difference to aupairs if they are coming over for a specific job.
Before the Eastern European countries joined the EU, we used to provide a letter of invitation for our au pairs, who were able to work in Britain/EU for up to 2 years as an au pair. I imagine something similar will replace the free movement after suicide Brexshit.