There have always been classic examples of children's entertainment that have also been watchable or readable for adults - Sesame Street reigned supreme in this category when I was a youngster, and there's plenty of classic kid lit that you'll find lurking on thoroughly adult bookshelves everywhere. Scott Nixon tackles the issue of 'piggybacking' in games in an interesting Gamasutra piece - what does it take to make a game that kids can get but adults aren't going to find eye rollingly bad?
Some games lend themselves to Piggybacking better than others, in large part because some games are inherently more fun to be an adjunct participant in than others. In general the hierarchy is consoles over computers, turn-based games over real-time and thinking games over twitch ....
Straight action games are usually spectator friendly only in a very limited "me-centric" way -- you watch, often cringing, leaning from side to side, all the while itching to get your hands on the controller and show whoever is playing how to do it right. As much as you want to help as a spectator, your warnings and suggestions will rarely yield any real benefits and are more likely just to frustrate the primary player.
You can't stick two half-decent Unreal Tournament players together and get one amazing player, but two sub-par logicians working together on an adventure game can easily overcome obstacles where one alone would be adrift.
From there, Nixon points to some concrete examples of how this piggybacking concept can work well in games, and some of the inherent problems with making children's games that can appeal to all ages. He exhorts designers of children's games to "remember there may be a literate and highly critical parent watching and judging your every move"; not bad advice for any creator of children's media.
Piggybacking: Gaming Across the Generation Gap [Gamasutra]