I don't think gaming has age limits. And I don't think a game needs to look "mature" to be great. One of my favorite games this year was ni no kuni.
But even lighthearted/family games need some depth to them. I have kids and I buy and play plenty of games with them. My daughter knows when she gets a shallow game wit repetitive mechanics no matter how good it's characters look(most Disney movie games). And from what I've seen this game is linear beat em up with limited combos and a gimicky growth/shrink mechanic. It looks to be fun for a while, but nothing that would hold mine or my daughters attention for very long.
looking mature and looking complex are 2 different things...i talked about the latter..which your second point also talked about, so i'll talk about that...the complexity of a game shouldnt matter when determining whether or not you gonna play it, that's what i'm saying...you should take a game for what it is, not for what it's not...when i play family/lighhearted games, i dont look for complex/tough game play..i'm expecting something simple and repetitive, since it's probably aimed at a younger crowd...on a side note: even complex games have repetitive gameplay, but that's another topic...
anyway, you say this game wouldnt hold yours nor your daughter's attention for very long, what is 'very long' specifically?...i ask because if the game is fun and yall play for an hour or 2 here and there, you not gonna notice how linear, shallow, and repetitive the game is cause yall gonna be too busy enjoying yallselves...
...i dont know if knack is fun, havent played it yet, but from what i've seen so far, it looks fun to me...it doesnt to you...that's cool....after i play it, i'll let you know what i think...