Kids can't talk to anyone who you don't add as a friend, and your child's friends list is managed by you and new friends must be approved by the parents of both kids. You can see at any time who they have been talking to, you can see all the images that have been sent and received, and there's no advertising. It's basically a walled garden just for your kids group of friends who've been approved by parents and nothing more. Kids can't even start a group chat or video chat with friends who aren't all mutual friends, so there's no way for your child to talk to anyone who you haven't approved.
It has stuff that kids like, like little games and filters they can use to play with their friends, which is clearly just to increase engagement (and screen time), but it's actually not nearly as terrible as you'd expect from Facebook (yet).
Messenger for Kids has been a godsend during the quarantine due to the fact that the kids want to do video chat but all have different types of devices, and it's nice because it's locked down to only the people you approve.
Now, is Instagram for Kids a good idea? We shall see, my guess is "probably not" but at least Messenger for Kids isn't exactly the nightmare you'd think it is based on the name.