When I walk the schoolies to school (so 5 and up), I don't hold their hands other than to cross the road - unless they want to, sometimes they just want to hold my hand and chat. If they were 3 year olds, and we were going on a waling excursion somewhere, I'd hold the hands of the ones I know aren't trustworthy! I guess parents know their children's 'levels' of understanding.
If it were in the city though, I'd be a bit more panicky!