I think there are a few issues:
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How do you prove kid A gave kid B measels?
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Why isn't kid B vaccinated? Because they don't need to be, group immunity. Well that is no longer true with anti vax so...
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Kid B then gives kid C measels, so kid B's parents are now liable.
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Your in small claims court. You have to prove damages. So you're going for loss of earning for an adult looking after the kid + pain and suffering. Is that payout going to be worth filing papers, legal advice etc.
You'd be better passing a law to mandate vaccines, but that won't be politically viable.
Just my thoughts - am not Canadian.