The rule is that they can’t use Blizzard’s social communication channels to engage in langauge that is not appropriate. If they are using Discord or anything that can’t be seen in-game, that is not something Blizzard has control over.
Again, they action based on reports of what other players see. Things in General chat, Trade chat, /say, emotes, whispers to you. Whispers privately between friends are not something they police. No more than in the real world. It is when those things cross the line to being in public or said to non-consenting parties that it is something they can penalize.
Just don’t think too much about it (I don’t think about what others do in the privacy of their bedrooms for example) and use all those tools Blizzard provided to safeguard your child!