Explaining the whole "side-games" thing is best done by telling them that KH is made up of two trilogies.

There's the Xemnas trilogy, and there's the Xehanort trilogy. Xemnas trilogy in 1,COM,2, and the Xehanort trilogy is BBS, DDD, and KH3. Skipping from 2 to 3 is like skipping from Revenge of the Sith to Return of the Jedi. Of course you'll be confused, to missed a New Hope and Empire Strikes Back.

It's a bit understandable when there are actual spin-off games like 358 and Coded that add side-stories, akin to Rogue One or Solo which don't really need to be seen but add some flavor and expand upon background events or background supplemental characters (Xion who?).