I bet the kids got sick from the paddling pool water. Mila even described it as "wee water", so either she noticed it was off colour or tasted strange.. or one of them peed in it and was scared to own up. The hosepipe probably had some water left festering inside, from when it was last used, and I can't imagine it being proper drinking water quality. The kids were squirting it around all over the place and no doubt ingested some.
I hope Aurora doesn't also get ill because Isla was kissing her loads. They're always kissing that baby. I find it OTT tbh....even without the contagious coldsore issue.