"It's people who deliberately expose their children to chicken pox (especially befor 18 months old) who risk shingles for them later in life. People who have never had chickenpox can't get shingles and you can't catch shingles."
That's not quite right. It is correct that you cannot contract shingles unless you have had chicken pox. But if you have had chickenpox as a disease (whether by deliberate exposure or not) or the chicken pox vaccine (attenuated live virus) you can get shingles.
Indeed the effect on patterns of shingles is one of the big unknowns of the vaccine - there have been concerns it will make shingles more common, but right now no-one knows.
NHS does not recommend quarantine for chicken pox. They do recommend isolation of the actual patient from diagnosis to pustules all dried up, but no quarantine for contacts, whether siblings, classmates or anyone else.
But if you choose to quarantine anyhow, it needs to be a full 21 days from the day after last contact with person with disease, or if sibling, 21 days from the day after the last pustule has dried (so you're probably looking at a clear month, given the spots usually take 5-7 days).
There is no point whatsoever in doing it for less than the full time it takes to cover the whole incubation period.