Parents must be aware of a hidden gotcha of the Pokemon DS series video games. These games do NOT have the ability for multiple kids to share the same game cartridge. My wife and I expected that all video games had the feature where kid1 can save his own game, then kid2 can save her own game while sharing the same physical cartridge. Not so in the Pokemon franchise. Each cartridge has one and only one "saved file" (as we call it in our house). If you have multiple kids they must either share the same "saved file" (which is no fun) or they have to have separate physical cartridges (which is expensive).
So yes, you have it right, if kid1 wants access to all the Pokemon in the X/Y series he has to buy BOTH X and Y. The games are identical, just the characters are different. Double Whammy on cost there. Then if kid2 wants the same, she must buy both games again. For those keeping track at home that's ~$50 * 4 = $200. Ugh.
Don't get me wrong, Pokemon games are awesome. Kids love 'em, their friends love 'em. Positive messages, engaging game play and all that, just be warned about the non-existent game sharing.