I'll say this from personal experience; having children play a musical instrument such as the piano when they are young helps massively with finger dexterity, speed, and muscle development.

Having children also tinker with electronics (and anything else modular for that matter) is also a great way to gain accuracy needed for minute and subtle movements.

These have added benefits over simple handwriting -- in that writing is a prolonged contraction of the muscle whereas playing an instrument and doing small scale work involves contraction-release and learning of pressure sensitivity.