The answer depends very much on the child's motor skills.
Basically, there are two different writing technology options here: rollerball and fountain pen. Rollerball pens are preferable to fountain pens when children has not yet developed good and automated writing motor skills. However, if children are already achieving good results with a pencil, can apply pressure well and write fluently and rhythmically, then the fountain pen is a good choice.
A learn-to-write fountain pen or rollerball is a writing instrument designed for the needs and anatomical conditions of junior school children. In addition to ergonomic grip zones, the features of school fountain pens range from special nibs for beginners and left-handers, to special nib surrounds to prevent blue fingers, to particularly large viewing windows for the ink level and increased leak protection.