Unfortunately, blocking of "all" subversive or age inappropriate content websites is virtually impossible short of, as with sex, abstinence, even on a home PC. There are some options in the Android marketplace; content filters, tracking/monitoring software, net nannys. The only prevailing downfall, and this is all of them that I've seen, is that they can be turned off by the phone user. Makes for a pointless exercise.
Beyond that, communication with your children is beyond vital. As a parent of 3 myself, I share your angst at the unwillingness of the carriers to be more proactive on this, and the apparent indifference displayed. No, I don't "blame" per say, the carriers, Google, Microsoft, or Apple for the problems this foments, but I certainly do suggest they should work more with parents and schools on an issue that will increasingly become more acute and epidemic.
On one hand, the knee-jerk, pat responses of "take the phone away", or "give them a basic phone" though obnoxious, and short-sighted, are accurate, while on the other, the carriers are actively pushing people into smartphones and are discouraging the purchase of basic phones, and THAT puts the ball of indifference back into the carriers court. I've been told by numerous reps (both CSRs and Tech reps) that the eventual goal IS to eliminate the basic phones altogether, so I'm hoping that some entity, be it a software company, carriers, Google, whoever, join in the fight to protect the children from things they neither need to see, nor are prepared for.
a long shot in the Wind if a Kid want to see what a 
kid wants to see it's hard to keep him from it.. 
