I am playing Pokemon TCG with my kids. My son has the Altaria GX card. The GX Attack is Euphoria GX which states "...Heal all damage from all of your Pokemon." Does the healing effect knocked out Pokemon (i.e. those that have been reduced to 0 HP)? Thanks!
No, KOed Pokemon are in the discard pile and no longer in play. In particular, KOed Pokemon don't have damage counters on them anymore, and they're not treated as a single Pokemon either (e.g. a Stage 2 Pokemon in play is a single Pokemon, but if it's sent to the discard pile it will usually be 3 cards, not a single Pokemon). Therefore this effect from Altaria GX will not revive them.
