This piece of light children's entertainment is aimed at the same market as all those movies that used to star Hulk Hogan or Arnold Schwarzenegger apparently sending up their tough guy image. The Nanny. Kindergarten Cop. That type of thing.
In Tooth Fairy, wrestling star Dwayne "The Rock" Johnson must serve time as a tooth fairy, in order to atone for having revealed to his girlfriend's child that the tooth fairy might not be real. This set-up is sort of a riff on the Peter Pan conceit, where if you say you don't believe in fairies, a fairy somewhere drops down dead. Only in this version, instead of a fairy dying, you become a fairy, so maybe it's actually more like The Santa Clause starring Tim Allen, in which Tim Allen takes his sweet time facing up to the fact that he is now Father Christmas. Funnily enough, Tooth Fairy director Michael Lembeck also directed The Santa Clause 2 and 3, so this is clearly his whole area.
Judging by the rules of the universe as set up in this film, The Rock is lucky it was the tooth fairy he targeted with his disbelief. If he had been reassuring a child that the boogeyman is not real, he would presumably have had to do a stint as a nightstalking killer, which would have stretched even The Rock's capacity for easy-going charm. As is, he gets to exhibit the same sort of oddly likeable screen persona that made it easy to root for him in his wrestling matches (especially when he would delay the match to make grandiose speeches about smacking down candyasses and smelling what The Rock is cooking).
Here, The Rock's character Derek Thompson is not a wrestler. Instead, he is a professional hockey player, albeit it a brutal hockey player who his coach uses mainly in order to flatten problematic members of the opposing team, rather than actually score goals. He is so good at this that he knocks many of their teeth out along the way, and is nicknamed the Tooth Fairy, which you might think is oddly coincidental considering he later gets turned into a tooth fairy, but in fact would explain why he's such a grouch about the whole tooth fairy concept. As nicknames go, it's not exactly up there with the likes of 'The King' or 'The Man In Black'. Or The Rock, come to think of it.
Ultimately, this movie will probably only be of sustained interest if the idea of The Rock in a tutu strikes you as a nice comical set piece. You see, he's a big macho guy, but we see him wearing a tutu, a garment not traditionally associated with machismo, and he does not look comfortable. If this does it for you, this film has plenty more where that came from. Billy Crystal and the UK's own Stephen Merchant also appear, but even their considerable combined talents can't elevate this cookie cutter demonstration of what a big studio thinks children want from their films.