I've double checked the information they've sent home and it definitely says the whole family must isolate if a child or member of staff from the bubble has symptoms. That is part of the condition of you sending your child back but it may differ from school to school/area to area.
I know when i intially read it, i thought it was a bit crazy as kids always gets sniffles and temperatures on their return to school and I'm not sure many parents could afford to keep having to take odd days off everytime a child in their childs bubble has symptoms until they get test results.
It is very clear our school do not want pupils returning en mass though, they have a very long and comprehensive list of conditions parents and pupils must meet, so i wouldn't be surprised if the isolation for the whole family is not a nationwide thing!