I discovered by watching them that, although their letters looked acceptable when finished, they were not forming their letters with the proper strokes.
Children see plenty of examples of well-formed letters every day.
What they don't see is the proper stroke sequence required to achieve the letters. The sequence is typically represented in penmanship books by numbered arrows, but that requires a level of abstraction that not many young children have developed yet. Options for input from the keyboard or onscreen buttons, and writing on 'paper' or a 'blackboard'.