The Babysitter At the Lake

Okay, I just have to rant about something for a minute. I know that my "youthful" appearance will be great when I'm 40, but I'm almost positive it's the reason for my latest irritating experience. I made a last-minute decision to take the girls to the lake yesterday for some post-VBS fun. I didn't have a swimsuit with me, but Noodle had one and I just stripped Hopers down to a swim diaper. There is a sandy beach area that gently trails off into the lake water, creating a shallow swimming area. Noodle is an excellent swimmer and jumped into the water among the throng of camp children that were there. Hopers preferred to spend most of her time wandering around the sand and eating snacks that I'd brought. This was fine with me. I stood in the shade of a bordering oak tree and watched my kids with amusement. Hopers would wander into the water (about waist deep for her) and Noodle would pick her up and splash around with her. Of course, she constantly fell down, getting her face in the water. I would walk out to her and help her get up if she needed it. At one point while I was standing out in the water after picking up Hopers, I realized a large woman was wading towards me after saying to her friend, "I'm going to say something!" The hostility in her look was all I needed to understand her point.
"Are you his babysitter?" she questioned me, moving closer.
I felt confused. Was she talking to me? I pointed at Hopers and shook my head.
"Are you the mom?" she said in a positively rude tone of voice.
I nodded.
"Are you sure?" she said, sneering.
I raised my eyebrows and nodded again. What was this lady's problem?
"Well I'm just shocked," she said and began making her way back to her friend.
I pretended to ignore her comment. She apparently thought I was some teenage babysitter who didn't know what she was doing!
A few minutes later I noticed she was talking with Noodle out in the water. I asked Noodle on the way home what the woman had said to her. It seems that the woman still hadn't believed I was the parent and was questioning Noodle as to whether or not I was the babysitter for her and her "baby brother". Noodle, of course, set her straight on both accounts. (Poor Hopers!) I am still feeling miffed about the whole incident. What makes people think it's okay to act that way?