My young teen worked at a local library through their student volunteer program, with school permission.
Also worked at our local food bank, with me present.
You won't find many places that will take underage volunteers without a parent present to supervise them, these days. I'm sure as a family you could volunteer on community clean-up days, or local park trail maintenance days. Your kids could accumulate volunteer hours that way.
Or, if they were part of a scout (or other youth leadership) program, they could earn volunteer hours through group community service projects.