Hi 'banana' and other respondents -
I did five years as a parent (father) helper when my DS started school, hearing Yr 1 / Yr 2 readers at first, gradually taking Yr 6 for gardening activities, and running an after-school keyboard club with Yr 6 children.
As I was out of work at the time, I applied for TA jobs in other schools, and then was ten years in an Infant school, mostly with Yr 2, but at various times also Nursery, R, and Yr 1. As some children didn't have a male adult at home, I was possibly the only responsible, sympathetic but disciplined man some kids knew.
I ran lunchtime recorder groups, had a percussion club, and coached children on percussion for the annual 'production'. I also did touch typing with Yr 2, and ran a computer club with Yr 6 at our companion Junior school.
When I retired I continued to give voluntary help in Primary and Secondary schools.
Re 'skills' :
I would have thought that any specialist skill not normally practised to a high level by class teachers could be useful, and might be welcomed, eg, music, dance, drama, singing, sports of all kinds, arts and crafts, gardening and nature, cookery, languages. Of course, Primary teachers have to take singing, music and sports activities, but sometimes are not that comfortable with it.
It is very satisfying when you enable a child to understand something it was struggling with, or achieve something they previously thought they could not do.