Am I wrong to be upset?

I’ll try to make this short..my brothers girlfriend watched my kids yesterday while I was at my doctors appointment. I have 4 kids (8,8,8 and 5.) When I pulled up to pick them up, all four kids were outside and she wasn’t anywhere outside. After I got out of the car, I asked my kids where she was and they said inside. I went to go inside and the door was locked. I thought it was really weird. I knocked and she came to the door like 30 second later. I was in a rush to get back home because we were having internet installed so I didn’t ask her about it. Later, I wrote her saying not to leave the kids unattended because you never know what could happen especially nowadays. She said explained that she was laying her kid down for a nap and that she didn’t do anything wrong. She said that when she was 8, she was allowed to do things on her own. I told her times are different now and she never responded. She’s now pissed off at me. I never “went off” on her, just said I didn’t like it and not to do it again🤷🏼‍♀️