Kids outside unattended

Would you tell your 6 year olds to go outside and play? By themselves? I was surprised when my DH told me that his kids told him that their Mom tells them to go outside and play and she doesn’t go with them and I’m being nice when I say they don’t live in a nice neighborhood. They live in Section 8 apartments and there’s quite a bit of crime. They share custody and because they’ve had issues getting along in the past, he’s hesitant to bring it up. For the next several days he will have the kids but then they’ll be going back to their Mom. How does he go about addressing this without it turning into a fight?