A spider which is new to an area is probably dangerous to children.
A person who normally pays little attention to spiders may have consciously noticed fewer than ten species. Any temperate-zone region is likely home to several hundred species of spider. So any given specimen stands a favorable chance of being new to you, without being new to the area! The number of spiders that pose even a mild hazard to humans is vanishingly small (perhaps 1/20 of 1% of all species) and, unsurprisingly, those few species are far more widely publicized than the harmless majority. So that specimen you found that is unfamiliar to you is therefore almost certainly nothing to worry about!