Sorry to hear that.
I can understand your friend being angry at the unvaccinated worker, but as you said the Dad also got it, albeit mild. So equally it could have been a vaccinated childcare worker who passed it on. That’s the biggest issue with this IMO, anyone can get it and pass it on. Unless people know they have it and then isolate, they won’t realise they’re spreading it either.
Equally you can be sicker than you’ve ever felt before, but at home and barely able to get out of bed for 3 weeks or more. Yes, you’re not hospitalised but not necessarily what I’d call mild. Your chances of being that sick may be less if double vaccinated and boosted, but that is of very little comfort if it’s you or a loved one who’s sick.