“A Monster Calls” marks the third 2016 film to feature a child’s relationship with invented creatures, joining “The BFG” and David Lowery’s “Pete’s Dragon” reboot in a trifecta of stories about the interplay of dramatic experiences and imagination. Considering how easily the material could turn maudlin, it’s especially notable that Bayona avoids overstatement. The monster may or may not be a metaphor; either way, it leaves a strong impression. One can only hope that Bayona, about to head into the universe of massive Hollywood tentpoles, will keep it up.