There are plenty of books and guides about running classes and workshops yes, but there are some general rules:
know who your audience is, age and ability
describe some learning outcomes, what they'll be able to do by attending
have a rought syllabus if not a timetabled one - you need a plan to start with
parents will want to know what's happening, especially if you're asking them to pay
do you need accreditation of any sort, i.e. local authority approval? we run children's workshops as part of our local community theatre and we have affiliated to the county council young people's/education department, also people running workshops for children have to be CRB checked, registers kept, documentation about youngsters on medication etc. It is quite a minefield (but you may already know this!).