I once asked my mother's advice for my selective mutism. But she said, "I'm also not good at talking. (So, you're not the only one who're not good at talking)"
She didn't understand my selective mutism. She was a reticent person, but hasn't suffered from selective mutism. She confused a reticent child with selectively mute child.
In Japan, when someone is troubled by something and whine about that, people sometimes said to his or her, "You're not the only one who're troubled by such things. Everyone is troubled by and has patience with such things." Some people may add, "Don't be a big baby!"
Such words sometimes ease his or her mind or encourage his or her to bear the hardship. But they also may trivialize his or her difficulty.
In terms of selective mutism, I guess a small proportion of children suffer from such difficulty. So, it's inappropriate to say to selectively mute children, "You're not the only one ..."
But few people understand selective mutism. So, some people may say such a thing to selectively mute children.
