This is written by the preschool-counselor me, not the high-schooler me.
1) If it’s a familiar environment (their home or a home they know their way around in) and they’re in shouting distance, I’d say fine (though I’d be neurotic and magnetically attracted to their cuteness and just go watch them anyway 🙂 but that’s just me).
2) As above, I’d probably end up on the floor playing with the kid, but as long as you’re eye is more than half on the kid, I’d do it. What gets to be the problem is when the watcher is texting while the watchee is toddling off into an unfamiliar hotel where it’s mere chance that walking towards her is her loving cousin (me) to help her. As long as the watcher is RESPONSIBLE, I think that’s fine.