7.5 yo totally fine. 3yo? Not.

I have 8yo and 10yo and I check in occasionally but they play on their own on play dates.

At 7.5, no big deal. It's not like it was a bunch of 3 year olds.

If she's old enough to leave your house and walk down the street to another house then she's old enough to play with her friend unsupervised for 15 minutes.

Last year I was preparing for a 4-day camping trip and I knew it was my last chance for a nice long hot shower. My 3 kids were home (9, 7, 7) and my older son had two friends over who were also 9. I told the boys I was in the shower, but as usual, they weren't listening. One of the moms came by, knocked on the door and then let herself in. She asked the boys where I was. My son said he didn't know! Then she asked if I was home and the boys said they didn't know! After a few minutes, I emerged from the shower and we all had a good laugh.