Irritated and Frustrated with Babysitters...

So, by my topic you can see that I'm not happy right now. I thought I found a wonderful babysitter that practiced good judgement. Now what I once thought is under question. Yesterday, when I went to pick up my son from his babysitter I decided to walk up to her house. While I was walking there I saw from a long ways off that Josiah was playing outside. Little did I know that he was playing outside with the supervision of a second grader. Yes, that's what I said, a second grader watching my now three year old son. I was livid and couldn't believe my eyes. I pay extra to bring him to her house rather than bringing him to a daycare. Isn't it common sense that I pay her to watch my child and not her son. All the possibilities went through my mind about what could have happened. Someone could have stole him, he could have got ran over by a car, or even a dog could have attacked him. Even after speaking to her I'm very scared about her judgement calls. If she thinks he's big enough to play outside without adult supervision, what other judgment calls does she make. So, I've been thinking of finding another babysitter. I know I should probably take my concerns to her, but my worry is that she will get upset and take it out on my son during the day. It's so frustrating not knowing what to do in this situation. The babysitter is such a nice lady and has a great family with wonderful kids. That's why I chose her in the first place. Maybe I'm being over sensitive and over thinking everything. I just hope I figure this out soon.