Xbox One child account can't play Terraria multiplayer online
Xbox One child account cannot play Terraria in multiplayer/online mode. When starting the game, "Online" option is greyed out and cannot be selected. If the child tried to join his friend's game, the game starts but then immediately displays an error message "The game will end due to the Membership Content settings of one of the signed in profiles".
The child has Xbox privacy option "Join multiplayer games" set by his parent to "Allow" and other games work in multiplayer mode just fine.