Feel the need to say something else here.
But will be scant will details so as not to out myself.
When my daughters were younger, they were friends with a group of siblings. The siblings had mum and step dad.
I am sure that on occasions my DDs were alone with the siblings and step dad.
At one point, stepdad was looking after a big house for someone, while owner was away.
He wanted [as far as I can remember, owner didnt mind] my DDs and his stepchildren and at least 2 other girls to spend the night at the empty house. I didnt see a problem with this as I knew the stepdad.
I did think it was a little odd, but wasnt concerned. However the parent of the other girls was concerned enough about it, to ask me what I thought. I said I thought there wasnt a problem, as my girls had been with this man often.
So, they all went.had a nice time [though I was glad that it went ok, as I had said it would to the other parent.]
Thought no more about it.
Then, about 5 years later, when my girls were a lot more mature, the stepdad came up in conversation.He had by this time parted from the mum. And they told me that they had never trusted him, they had always felt a bit funny around him. They all thought the same. 
So, actually,I suppose, and I dont know how to put this, you might consider asking your DC, if old enough, what they think about their friends parents or guardians.
There may well be one or two of them, who are a bit uneasy. But may not be saying anything.
I never thought to ask mine, probably because I considered my own judgement good enough.