I thought it might be helpful to go back to the original post which
clarified that this question is being asked in relation to work in a
children's history museum.  It seems to me that the emphasis in such a site
should not be on the importance of teaching history, or even learning
history, but on the importance of developing research skills in young
people so that they can use these tools to pose their own questions and
follow a path toward answering them.  If I was working at your site, I
would frame my mission statement to include something like the following:
 
"The museum exists to provide opportunities for young people to develop
skills to explore their own histories.  In recognition of the fact that,
from time to time, an artifact may focus a person's fascination with former
times, museums are environments where one's imagination of the past may be
enriched.  The museum's role is to support investigations of the past in
the hope that, in some way, they may contribute to a more complex
understanding of the present and form a basis for imagining our shared
futures.
 
Judith Mastai