Gnome children are young gnomes found in the Tree Gnome Stronghold. Like adult gnomes, they can be killed or pickpocketed. All of them use melee unlike their adult counterparts. Usually they are easily killed in one hit. The fact that they are children and attackable is a subject of controversy among the RuneScape community.
When talking to one of the gnome children it's possible for the child to warn you of a new world coming from the underground. It is unknown if this is a possible reference to Arposandra and if so why children know anything about it.
It is also possible for a gnome child to offer you a king worm, for which can then be used for a gnome cooking dish.