I think it is reasonable to have kids play outside or outside inside freely, absolutely. But Hollanda said the weather was really hot and I'd be wondering how long my kid spent outside in the heat while the childminder was tanning.
While it isn't so much an issue here, in general I think there is a difference between free play where an adult is paying attention and free play where the adult is engrossed with another activity.
While this was likely an ok situation, I don't think there is anything wrong with asking about it- it is Hollanda's child and if she has a question about his care, it is her right to ask it.
ETA: I used to do this kind of care for a couple of family friends when I was younger. I did not spend every minute of every day directly playing with their kids. I would not, however, have a movie marathon or read a book unless the kids were napping. I might sit on the couch and just watch while the kids are playing with one another, but I avoided doing things that might give the impression that my priority at the moment wasn't the kids that I was responsible for.