It's each to their own with what they allow.
My rules might be crazy to some, but that's ok. Others rules to allow sleepovers, especially at a young age are crazy to me.
My DC have lots of fun with their friends, but they don't need to be sleeping over anywhere. They're not missing out in my book. Under my roof, with my family. No questions whether they might be exposed to anything by a parent, peer on peer abuse, older siblings, boyfriends/girlfriends, random people coming to the home.
So, I'm happy with my choice.
As I say, when they get older, they'll be going out with their friends. That's absolutely fine with me. They can go round to their friends when they're older, when they will be more confident and know how to handle situations, have a phone and even be old enough to walk out of situations if they feel uncomfortable.
This whole obsession with sleepovers/playdates is bizarre to me. They see their friends at school, see they some weekends, at parties and during the holidays at activities etc. But each to their own.