It's about transmission chain - and while it makes sense in theory, its very subjective.
Child's nursery worker is Covid positive. Child is therefore exposed in her bubble and so needs to isolate. This is the same 'rule' as 'if anyone in your household is infected' has been till now.
However, because there is no way of knowing that transmission to child has actually occurred (and because we really would destroy the economy if we ran this to its nth degree) until she either shows symptoms or tests positive herself, there is no transmission chain to her parents, who are therefore not required to isolate.
It's a one-step only requirement, otherwise you'd never be able to lift any form of lockdown at all as the alternative is:
Keyworker - child - parents - all parents co-workers and contacts over last 14 days - all parents coworkers and contacts' household members - all parents co-workers and contacts households co-workers and contacts over last 14 days and on and on and on, as you'd have to assume positive transmission at every possible step.
Common sense is needed, both ways. In this case, given she can work from home, it would be sensible for child and parents to all isolate and be tested over the next week or so to see see what happens, but they are correct in saying it is only the child that needs to isolate.