New Super Mario Bros. Wii

I felt I had to put that in the title, as it colours my review heavily; I spent the majority of this game in at least 2 player, with 4 players happening regularly, with a mix of players - my wife, 26, and a decent gamer; her 12 year old gamer cousin; his 10 year old mildly enthused sister; my 6 year old son; and, finally, at parts my 4 year old daughter. Both of my kids play a lot of Wii games, and know how to use the system well.
 
I bother to point that out, because a lot of parents will buy this for kids. After all, it says 3+/E For Everyone on the box, how hard can it be? The answer is, very. The big issue with the co-op play, which should in all honesty be there to make the game easier, is that at many points the design lends itself to single player gaming. There are a lot of sections where multiple players artificially ups the difficulty to a massive degree, such as the entire last 2 worlds. Though to get there will demonstrate heavily whether you can play this, the fact is that's a quarter of the game that is to be honest pretty inaccessible to multiple players who aren't awesome gamers. 
 
In fact, throughout the game, you would need a team of pretty amazing co-op players to make anything above 2 player work. The support systems for carrying lesser ability players (bubbling, carrying, even Yoshi swallowing) are actually very unhelpful on regular occasions: bubbles can be popped by accident to lead to unavoidable death, and if everyone is in bubbles the attempt at the level is over; carrying means a player cannot jump as well, and the increased height often leads to unexpected death, and if the carried player shakes they leap off, and often by accident, and to their doom; Yoshi swallowing has the same issues as carrying, but adds the fact that most Yoshi levels are designed for all players to be on Yoshis at all times. 
 
This, too, brings up another issue: many levels are designed to make the most of the new and unique equipment. For instance, there are at least 2 snow levels designed to use the Penguin's belly sliding ability, and the last level's ending part is heavily skewed towards the Heli Hat. Without said equipment, the difficulty spikes heavily, often to a ridiculous level. For instance, on one of the ice levels, the Penguin's belly slide allows you to slide across platforms with a small gap inbetween, but without it, you have to jump across slippy, thin platforms with a very small margin of error. 
 
By world 7, it basically became a case of me and the 12 year old gamer playing the game, with everyone else having to bubble away to not hinder us and die, which they still did by accident a few times per level attempt anyway. It says a lot that, by the end of the final world, nobody had used less than 40 continues, which is roughly 200 lives. Around 15-20 of those, each, were the final 2 worlds. By world 8, the other players were bored of spending all their time in a bubble, and so it became just the two of us. With two, experienced players, the loss of lives slowed, and we made easier progress. It's weird to be able to say that, in a multiplayer game, it's easier to have less players. 
 
So, in summary: young kids, despite the age range, shouldn't be playing this unless they're exceptionally gifted, as to be frank it gets very hard after about world 5 and expert level by world 7. The multiplayer aspect does not help this, at all, and in a lot of ways makes it worse. As an adult and gamer of 24 years, I found the last world incredibly hard, and wouldn't recommend attempting it to anyone who isn't supremely confident of their Mario skills. The last level, in particular, is a real throwback to the original Super Mario Bros., and is just as hard; and then it turns out that the last level isn't over, and what follows is even harder. To say "this is a game for kids" is missing the point somewhat; although there is nothing objectionable in the content, the gameplay itself is just not open to younger players who aren't gaming savants. As such, as a parent playing with kids, I would say avoid.