It all depends 'where' they are playing.
If in a Playground with other Children, then absolutely not.I have seen some pretty full on kids, with other's kids and some of them end up hurt.
Some even at 3 & 4 years hit, slap, bite...You must be there watching your own child.
If in your own backyard, and you know it is a safe environment, then mum or dad can let them play, while of course keeping an eye on them every few minutes.
I really don't myself like a child unsupervised at all, unless they are say 10 onwards.Even then,I feel these are very different times, to when I was younger.
You now have people who will take a child, even from the close proximity of the parent.
I think to be safe, watch your child.