I used to stick up skittles. Helps if you know someone on the team.
Best bet in a small village imho is to print up some a5 leaflets for car washing. These handwash things are popping up all over the place. If your strapped for cash ask your olds if they would like to 'invest' in your business. To buy sponges, buckets, shampoo, shammies and polishing stuff. You can charge loads of money for a full valet but you will need to work hard. Cleaning the body, wheels, windows and then hoovering and polishing the interior.
Worth a go even if you only get one £25 customer a week. Remember that people will talk about it too so do well an they will come to you!
Home that's some help
Sorry but do you really think anyone is going to hand their nice shiny car to a 13 year old?
Maybe once they've learned to brush their teeth properly they'd be able to learn how to valet a car........