The rapper pulled the ultimate parenting move this weekend for his 7-year-old, Sire, by renting out an entire store in New Jersey -- only one of two locations open at the moment -- and closing it off to the public while his boy went around and picked out anything that caught his eye and letting him take it all home as his gift(s) for the holiday.

Basically, a personal shopping spree for a kid ... which became even more of a custom experience with Sire's name and mug incorporated into the presentation. A decal on the ground read 'Sire's Toys R Us' ... not even Charlie Bucket could've dreamt of this.

As for how much all of this cost ... we're told Fitty dropped a pretty penny to give his son the best Christmas ever -- about $100k to have this place to himself and Sire.

Hell, it looks the cash went so far as to pull Geoffrey the Giraffe out of retirement -- who posed with 50 and Sire in a sweet photo-op. You'll recall, Toys 'R' Us temporarily closed up shop last year -- but it's being brought back, slowly but surely, under new ownership.