So I was musing about Posiedon's note in SoM (as you do) and realized that at first glance it's just about Thalia, it could also be about the di Angelos.
Poseidon would've known about Nico and Bianca, because there was no reason to hide them pre prophecy.
Percy gets the note in the summer before he turns 14. About 6 months later, he goes to Westover.
Now, admittedly I have no evidence of this, but I imagine that suddenly beginning to age again, after about 70 years of not aging, might make someone a bit sick. So if you account about a month to 1.5 months to adjust and recover from the Lotus Hotel, it would probably put the di Angelos starting school around mid-late August to early September, a normal time for school to start. (At least according to my American public school experiences)
So about 6 months after Percy gets his crappy yearly contact from Poseidon, which lines up to when Nico and Bianca mightve left the hotel, they meet Percy and Thalia.
I don't have much of a point, just a long ramble to point out the fact that Hey, maybe there could've been more to the note than just thalia reappearing.