Like Bruce Wayne searching for the truth behind his parents' death, Claudia and Jamie set out on a single-minded quest. But while Bruce Wayne becomes the hero we know as Batman, Claudia and Jamie just become serious art museum loiterers in order to be close to the angel statue. They need to find out the truth about the angel statue, something that only Mrs. Frankweiler holds the key to. Their quest for the truth in From the Mixed-Up Files of Mrs. Basil E. Frankweiler isn't even about bringing the information back to museum officials, or being lauded as heroes in the newspaper (though we have a feeling Jamie wouldn't have a problem with that). They are in this just to discover the real, unadulterated truth.
For Claudia and Jamie, it's more about the quest to find out the truth about the angel statue than the actual answer.
Mrs. Frankweiler keeps one truth from the Kinkaid kids at the end—that her lawyer is their grandfather. She does this because she already gave away the angel secret, and she always needs at least one secret.