What makes a unit spend several turns without regenerating mana or hit points (one turn should be roughly the equivalent of a couple of months, considering the children's age), then suddenly as soon as an enemy is presentthat same units starts regenerating like crazy.

I believe the mana regeneration should be constant (a little faster than usual) and the hit points regenerating should be triggered by something: maynbe the fact of being hit. Otherwise it just makes no sense that if I need to regenerate wounds I can just engage in a battle and run around wasting turns.