I played Sonic Mania recently with my seven year old sister, and at first I played as Sonic and she played as Tails, but she wanted to be Sonic at some point, so we swapped controllers. That's when I had a breakthrough - when you want to play Classic Sonic with a less experienced player, always pick Tails.
When the "younger sibling" is playing Tails, they:
-Will get bored/frustrated as Sonic zips around and leaves them behind for several seconds at a time
-Will want to play as the objectively cooler character, Sonic
-Want to try out Tails' flying ability, but can't because of the aforementioned Sonic zipping around
When the "older sibling" is playing Tails, they:
-Can demonstrate to the less experienced player how to do things like spindashes or how to defeat certain enemies/light platforming puzzles without worrying about leaving said player behind
-Can lete the less experienced player explore the levels at their own pace
-Can help the less experienced player cross large gaps with Tails' flying ability (gaps are already hard for platformer noobs since they don't immediatly understand how to move and jump at the same time, and even harder in classic Sonic when you can't just jump and press a direction to achieve a similar result).
-Can let the less experienced player be Sonic
I think I said this a while earlier but didn't elaborate because I was on mobile. But yeah, if you want to introduce a younger/less experienced one to gaming, I highly recommend Sonic 1-Mania, but please, for the love of all that is holy, choose Tails instead of Sonic.