Neither mother nor baby should feel "done-to" by the other.  The mom can
gently and firmly make the rule of "no twiddling or picking" and remove the
baby from the breast if she persists.  My toddler tried to nurse while
putting her toe in my mouth and persisted in trying this stunt for several
weeks.  Usually the toddler is bored and uses the opportunity to see how many
things she can do at once, and can't reach anything besides mom. (Maybe
training for being able to talking on the phone and read LactNet while
nursing a baby later on?) The mom must be prepared for a worst-case: weaning
if the kid can't separate nursing from picking.  However, I've never seen a
case of persistent mauling by a nursing toddler when mom is firm, loving and
consistent in stopping the mixed behaviors in my 24 years experience.
Linda Smith, a.k.a. Granny Zealot