Surely the place (if there is one) for a formal risk assessment is in making a generic RA of the activity itself, and in doing so make a list of the required controls to make the activity safe?
Then, when it comes to actually doing it, you check that the controls are in place, and if so, you do it; if not not, at least until you have reviewed the RA.
Those of us involved in RA for youth activities - schools, cadet forces, &c - do it like that, or we'd never do anything.
Tim