Proper youth work is a serious profession that requires training and qualifications. Free or cheap baby-sitting for middle class parents is something that most people could undertake and requires little more than a bit of common sense and a DBS check. The correct policies and procedures are important to safeguard all participants. Many county or district councils will provide a package of support for community led youth work, including resources such as training, mentoring, occasional visits from a qualified youth worker etc. As with any such activity, the insurance company might impose specific requirements, so it's always worth checking.
I haven't employed a qualified youth worker for a few years now, but you'd probably be looking at a rate of around £25 per hour for this sort of casual leader role.