To promote the movie in Japan....it gets a keychain.
This is a little figural keychain of Baby Movie Sonic, still in the package on the promotional card. The date, 3.27 is the date it'll release there. (But why write "Fri." in English? Aren't the days of the week there called something else? Would it confuse people?) It has a beaded chain at the top, and looks like the copyrights on a blue plastic tab that hangs with it.
Why choose the odd flashback baby? Maybe because Japan is known for liking cute things, or possibly the combo of that AND "It's baby" so it NOT looking like Sonic, but rather a "Sonic-like-creature" is ok/more palatable than the adult-form thing in the movie.

This gets a tag Japan because it was released in Japan with the movie first. In september 2021 it will get an international release, you can get it as pre-order bonus on Sonic Colors Ultimate.