This anime really had a chance to redeem itself and chose not to. The biggest problem in 91 days is a complete lack of character depth and development, so a special should be used to give these characters more traits and more likeable. But the special did nothing. Seeing some of the characters as kids isn’t gonna help the watcher understand who they are now, at least not with how poorly done this was.
The special is so forgettable and just lifeless. Don’t waste your time if you are feeling like 91 days left you incomplete.
If you loved 91 days and just want more character content, sure, have fun and watch it. You won’t really see anything that changes your perspective on what you saw in the original episodes. But I guess if you love the characters this would provide you more content.