My daughter is also twelve and has demonstrated the ability to care for children in babysitting situation. One of the ways we’re giving her experience sitting w/o having her bear the whole responsibility is by working for the church. There are groups that have got together at the church and hire small groups of teens to babysit. By being part of that she gets a chance to be in charge and if anything came up that was beyond her ability she could get the parents. If a family that we knew & trusted with 2 or 3 children hired her for childcare jobs, we would allow her, but for the first couple of times I would make sure I was going to be home so that if she needed assistance she could call for it.
We have a favorite babysitter who started sitting for us when she was just shy of 15. Her mother has trained her to babysit. She took her to first aid and cpr classes. She talked to her at great length about the things your child probably knows … age-appropriate toys, diapering, safety issues, etc. She taught her to clean up, put food away properly, and wash the dishes. She went along on her first several jobs to observe from a distance and watch how well she was able to manage the children. I would advise that you go over these types of things with your child. I have been quite surprised by the holes in my children’s knowledge and by the holes in the knowledge of some babysitters I have hired.