Aren't sidewalks and streets public spaces? Unless you're on private property (that you own), you can't legally slap someone, nor can you slap their phone out of their hand.
There are legitimate privacy and rights concerns to the legality of filming in public spaces (especially in this era of "big data"). None of them are exclusive to children.
Also note that parent was discussing the legality of filming, which is independent of the... interesting social moires you espouse.
In fact, that's underselling it. If it's my child's leg or your life, you're going to die. That's why I don't get myself into this situation in the first place.