My daughter is babysitter for our two young sons (not that we have the energy to go out much!). She is 15 and will be starting Year 11 in September. She is not a precocious 15 year old and very trustworthy!
PM me if you'd like to meet her.
Otherwise, when mine were at nursery, some of the staff used to offer babysitting in the evenings or at weekends...ditto with school holiday club...worth asking.