Translation:To educate a child is difficult work.
Water is not countable, we don't say «I drink the water» or «I drink a water.» The word "work" is used too with this uncountables meaning.
When an action is short or made once, then the action with -ad- is long or repeated. "Salti/kanti" is one jump/song, "saltadi/kantadi" are several jumps/songs. "Eduko" is already a more or less long action, e.g. for 1 child. Then what is longer? "Edukado" then is a constant work of eduaction, e.g. done by teachers