Spelunky is sort of like Indiana Jones meets Pitfall. You're a cave explorer, treasure hunter or perhaps an archeologist -- each level is randomly generated and no two are identical.
As you progress you move from caves to jungle to iceworlds etc, and you can battle the creatures on each level (bats, spiders, snakes, frogs, skeletons, vampires, piranha, cave men, aliens etc...), loot treasures, buy items from stores inside the maze that will help you in your explorations and rescue "damsels" (a dog, a girl or a guy) for a kiss that restores some of your life energy.
Of course, you can also sacrifice the damsels for special loot from a pagan diety called Kali, and rob the denizens.... so...while the game allows for positive role models -- it also allows you to be...more of a Belloq character than an Indiana Jones type if you prefer. .
It has cartoon violence and a small amount of blood -- and if you dilly dally and take too long on a level -- a somewhat scary ghost that will stalk you and kill you instantly if it touches your character.
The game is very challenging -- difficult even, and can be frustrating even for adult gamers -- but it's also very fun, addictive and if you are inclined to strategy games and puzzles and have an agile mind for impromptu problem solving you will find it rewarding!
My 8 year old absolutely loves it (but is terrified of the ghost) and can't seem to progress beyond the ice level...but it hasn't stopped her from trying...over and over again.
It does allow for cooperative multiplayer play -- but be prepared to die a lot! You have to have incredible team management skills or be semi-mind readers in order to survive together as a group for very long -- so if your kids are the kind to become very angry or frustrated by apparent ineptitude in others -- it's not the best multiplayer experience. I could see it degenerating into sibling squabbling due to the intensity of concentration and immediacy of problem solving involved. It's VERY HARD to play as a multi-player game -- but again, it's fun to do so.
Gameplay wise it's a tricky game and ideally suited for kids that love puzzles, strategy and can think and react quickly. Content wise it's fine for just about any age -- although that ghost is a bit scary. I think it's ideal for kids 8 and up -- the more experienced they are with puzzles and platformers the better.