This being a show meant for kids, I defer to a critical opinion I overheard right afterward, voiced by a child not yet 5: “That was really cool! Really funny too!” Coauthored by Mario Baldessari and Ethan Slater and directed by Jon Townson, How to Catch a Leprechaun weaves a bit o’ magic in forty minutes from eight merry tunes and four lively characters. If the gleeful sounds coming from the youngsters at the 2 p.m. matinee I attended yesterday are any indication, Keegan has found a winner for the wee ones. Plus there’s just enough over-their-heads humor to land some choice chuckles for grownups.