I'm going to leave my daughter's friend at the park
Why would you leave her at the park?
I don't feel like watching over her anymore at the moment.
When you're babysitting, you don't get to decide you're bored and abandon the child. You need to be responsible now, and I think it would be best to decline future opportunities for babysitting.
She's a human being and children need to be taught responsibility because they aren't like dogs.
There are better times and places to teach a child responsibility. Leaving someone alone in a park does not teach responsibility. Telling them why you need to watch over them and how you do so would teach responsibility.