I volunteered with Beanstalk (now absorbed into Coram) pre-pandemic in a local primary school. You work with the same children throughout the year and really form a bond. When I did it wass was two sessions a week, 30 mins per child, three children but that may have changed.
The charity supports with training etc and also if - as happens - the children disclose something to you then as well as following the school's safeguarding procedures you have someone to talk to there if needed.
There are other charities where they organise it differently.
I would go through a charity again as so many schools are so busy - it helps to have the external support, structure, a chance to meet other volunteers, etc.
One thing - I found it hard to take term time holidays as the children really minded if I changed my days or wasn't there - some have little stability in their lives. That's one reason I haven't gone back to it, as well as the level of emotional commitment given they do bond with you, some had difficult home lives and so on.
I also volunteer for a large national museum. I work a lot with school groups and families, mostly on projects run by the learning team. A lot of ex-teachers in our ranks (I am not one). It is much more flexible as you can do weekdays, weekends, term time or hols, just sign up for the shifts that suit you (there is a minimum commitment a year but it isn't onerous). And a choice of projects and roles including helping at holiday workshops and delivering talks to school groups. And being part of a team. Your MIL might enjoy something like that too?