How you respond kind of depends on the relationship you have with their mum. Also sleepovers are fine- until they’re not. There is a risk, even with good people.

I would be making sure I am 100% able to drop whatever I was doing and go pick them up if needed, and if the kids have their own means of contacting me, that does add an element of safety.

Having said that, my line is when the kids are with me it’s 200% my decision what happens. Not least of all because I’m close to no contact with their mother, due to her abusive nature. I’ve learned I can’t work with her on issues like this so I ignore her and use my own judgement.