Time Out says
A boy discovers he comes from a line of monsters in this enjoyable family cartoon
About a boy who suddenly transforms into a hideous red monster at a certain age, this animated film is clearly trying to sneak a warning about puberty into its kid-friendly adventure. Lucas is a slightly geeky schoolboy who’s desperate to go to parties rather than listen to his overprotective father. Turns out dad had reason to worry: Lucas has a hereditary condition and temporarily transforms into a beast at his ‘Carrie’-style prom. He flees town, ending up on the magical Monster Island. There he can live among his own – but with a mysterious kidnapper on the loose, there’s trouble brewing.
Monsters aside, it’s a familiar coming-of-age arc in which both parent and child learn something about liberty, family and change. Characters range from annoying malapropism-prone conjoined twins to amusing archetypes like the officious cop and his dim-witted sidekick. While it’s not up there with Pixar, ‘Monster Island’ is a moderately entertaining kids’ film that’s well put together by its Mexican creative team. And there’s a little something for parents, too: not least the assertion that they might, occasionally, be right.