I have the perfect way of restricting my kids from certain apps: I sit with them while they're playing, looking, laughing. An added benefit is that I can play, look and laugh as well. And because we're all crowded onto the same couch there's a fair bit of cuddling as a by-product.
Your point is well taken but it's not always possible. My 7 y/o loves the iPad and I purchased it in part to share with her and to enable her to use it on her own. I've been consistently disappointed by Apple's design in this regard. Either they're completely oblivious to the fact that an iPad (unlike a phone) may not be a purely personal device used by a single person or the company simply believes the "solution" to the problems raised on this thread is to purchase an iPad for every member of the family.
Even apart from questions of inappropriate content and security, I would like to enable my daughter to have her own set of apps organized as she wishes. I can set up a single home screen for her but I cannot limit her access to that screen. Nor can I label it in any way that gives her a feeling of "ownership." Even worse, if there are apps that are shared with other members of the family I cannot place the same icon on multiple pages. Angry Birds can be accessed either on her page or one of mine. It can't be both places. My wife, daughter, and I all have individual email accounts. While the email app can handle all three it cannot be configured to restrict access on an individual basis.
All in all, the iPad is a wonderful device for family members to share. The fact that Apple has failed so conspicuously to provide the tools necessary for this purpose is a shame. Considering that my $600 notebook provides the option for multiple user accounts with individual UI's, it's ridiculous that Apple has failed to provide similar functionality for a similar device.