Review: ‘The Lightning Thief’ inventively reveals the demigods among us

You think your dad (or mom) is a deadbeat? Consider the plight of the young characters in “The Lightning Thief: The Percy Jackson Musical,” a thoroughly endearing family-friendly musical at the Longacre Theatre. Each of the six youngsters in the show has but one fully engaged parent, if that. The other parents are variously absent, negligent, self-obsessed, entirely unknown or downright dangerous. Also, they are Greek gods. Which is no excuse. After all, you’d expect more from a god than a human, wouldn’t you?  Based on characters created by Rick Riordan in a series of popular young adult novels, the…