Well KQ7 is clearly an adventure game.With magnificent cartoon graphics i believe its a classic and can be played forever and ever.With the traditional kq series puzzles,based on many lore fairytales.Excellent sound and speech and relatively easy to play.Unfortunately it is a game made for teenage and younger girls.
As for KQ8 is a HUBRID of Adventure with RPG.The caracter is leveling,and the game is based on battles with swords and shields.Relatively good graphics and problematic gameplay mechanics that can be used to it if you seriously decide to play the game.And its a good game that's worth playing but has absolutely nothing to do with the previous KQ series.
Buy it and let your kid play KQ7 while you play KQ8.For the price its a definity buy!