My children rented a kid’s movie from Redbox sometime between 6-10ish years ago that we are all vaguely remembering. Young male maybe middle school age, has female best friend. He’s kidnapped by a blue troll/goblin who does some weird dance sequence in the sewers. We think we remember the blue creature never speaks.  Blue guy winds up being a good guy. Lots of potions in the film. A truth potion is taken by the boy at some point, too. So at the end, back at their school there is a chihuahua that belongs to an evil witch like lady who gives it a potion to drink and the chihuahua turns into its true form of a dragon type thing.