We've had similar, I got symptoms and tested positive on day 10 of quarantine, which I was in due to DP testing positive previously. Quarantine was calculated from last 'close contact' with DP, which was just before he had his positive result and went into isolation.
We're abroad but after my positive result, the local track&trace called me and since I hadn't been isolating from DC and been having normal 'close contact' with them until I developed symptoms, DC had to go back into quarantine as ordered by Track&Trace, calculated from last close contact with me. So DC who is the only one who never had any symptoms and always tested negative, was in quarantine for nearly 20 days.
OP it might be that Track&Trace will say similar in your case once they contact you, and order the other DC to stay at home for now for a renewed quarantine period.
However there is some logic in them going back to school too. If they haven't caught anything so far from your DD, chances are they have some natural immunity to it, or they've had it already asymptomatically during quarantine or before. They might of course still get ill, and indeed the lady from Track&Trace told me it is often the youngest in the family who catches it last and therefore has the longest time off school, but on balance the chances of them still getting it now (and potentially infecting others) is perhaps not so high to warrant extended quarantine and missing school and everything.
Meanwhile (pending instructions from Test&Trace) I would be sending the other DC to school as per guidance, but I would also be doing home tests with them if at all possible, and keeping a VERY close eye on potential symptoms cropping up. As your case and mine show, it is very possible to only develop symptoms on the last day of the 10-day quarantine - there is a reason the quarantine period is 10 days!
On another note, I hope you are not too unwell and able to manage your own isolation in some reasonable way! All the best for the coming days, get better soon!