It’s not that baffling. Incubation period and children being asymptomatic suggests that the child wouldn’t show any symptoms or may not test positive within 7-10 days of the other family members contracting. This is why schools are currently not safe. With adults, it’s somewhat easier to tell who has it because you would probably show some symptoms. Children can contract and pass this on to others who then do the same without anybody knowing about it, then bring the virus into the family home, or the supermarket, or the restaurant.
Her other kids have tested positive - they're 7 and 10.
There's lots of discrepancies with people testing positive one day and negative the next (see why Scott Parker and Ralph Hasenhuttl are now able to come out of isolation despite it being nowhere near two weeks). Similarly, the City players who tested negative on Xmas Eve and then tested positive 2/3 days later.
There's no set rule of thumb.
I've not even mentioned schools, just thought it was interesting.