How much of an effort should we make, given that they aren't complaining?
This depends upon the child, the opportunities for challenge outside the classroom, how comfortable they feel socially, how comfortable you feel with
advocacy, etc.
Is it unfair to them that they're not being challenged?
Yes, unfortunately kids are shortchanged if not challenged... see the thread on
What Kids Don't Learn. If you can provide sufficient challenge outside the classroom, that may offset the lack of challenge at school.
Would they be more "successful" if they were more challenged in elementary school?
Yes, the idea of challenge is not necessarily to make them prominent, but to increase the likelihood that they will gain the
skills which are learned through effort, struggle, and challenge. Having these skills may make it less likely that they will underachieve.
