Five Nights At Freddy's Age Rating Confusion
Solution 1:
Five Nights at Freddy's is a game series that has gained some pretty huge traction with younger children. Largely this is from YouTube channels that target their videos at children of about this age, just about every YouTube channel has done a video of themselves playing a d reacting to a fnaf game.
Is it an 'adult' game? It deals with some mature themes such as child abuse, murder and predators. But it's generally all implied, there is no detailed graphic content and any gore/violence is depicted in low quality cartoon like segments (borrowing an art style from old Atari 2600 games if that helps).
There is only one version of the game series, all of these games are horror titles and follow the same story line. However the popularity with younger audiences due to its massive impact through YouTube has spawned toys, pyjamas etc as you've noted.
The mature themes in these games are generally pretty subtle, and the unsubtle parts of the game are just simple jumpscares (where the enemies in the game jump out making a loud noise to startle the player) and a general feeling on tension from playing the game.
Bottom line, there is very little in these games that can just be given an age rating. There is no realistic gore, nor sexual content or anything like that. Whether you think your child would be safe enjoying this series is largely down to you knowing your child. The series is similar in 'scariness' to the early Childs Play films - corny and goofy, but if you find that kind of thing scary, you're better off leaving it be.
Solution 2:
The ESRB doesn't seem to have reviewed FNAF to give it an age rating.
But here on this steam post, seems to be a breakdown of what the rating would be, and why. https://steamcommunity.com/app/319510/discussions/0/412447613570515249/#c412447613573825769
if the esrb rated this it would probably be "T" because the eyes on the game over screen would count as gore; the esrb doesn't take scariness into consideration as it is subjective: if a game has blood then that is a fact, but if it is scary then that is an opinion; since some people find the game scary but others don't, they would not take scariness into consideration; fnaf 3 would definitely get a T because and the esrb has a rule that any game with in it gets at least a T.
Common sense media also seems to go for a Teen rating https://www.commonsensemedia.org/app-reviews/five-nights-at-freddys
But as the comment by @Ave said, kids will often play/watch stuff not caring about the rating. And it can be fine most of the time.
You know your kid; will jumpscares and a little bit of gore prevent him from sleeping at night, or will it be just fine?