I don't know whether it's okay or not. A teenager is more likely to be able to spend that downtime on something useful. Children, not as much. Whilst, I understand - more than most people - that we need alone time, I think that parents spend a lot more time on their phone, be it facebook, reddit, twitter, instagram etc, than they would like to admit.

I am sure they could sneak - if they already don't - some time on social media while the kids are playing on their own. For a kid, time with their parents is integral for growing up and having a healthy relationship with them. Most kids however don't get to experience it.

Additionally, time spend on facebook and social media has correlation with depression and stress. I don't think this is the best environment for a kid to be brought up.