Now that they're older (my son is 12 now) and really do have the sophistication to understand this distinction - and you're right, a 9yo or 11yo doesn't - they understand why we did that. But the key is, your 9yo and 11yo trust you, and should continue to do so. They don't care what you answer when signing them up for Google Mail - they don't even notice discrepancies of that nature. In the case they do, you tell them why you're doing it.
Google did most certainly make Alex cry - in exactly the same way that his hard drive crashing would have done so. Sure, ultimately it's the parents' fault, in a way, for letting him use fallible computer systems, and that in itself is probably a good lesson, but Google most certainly shares the blame here for boneheaded policies that don't even give parents the option of interceding. And the reason is the same as Google's reason always is: it was easier for Google.
The problem I have is with telling your kid: go ahead sign up and lie about your age. There's a huge difference between the parent taking the step of creating the account with a password known by the parent and telling the kid to go ahead and lie, which is what a lot of (probably childless) commenters suggest.
It's important to be clear that you're lying to a computer system, not another human being.
That will likely make parenting an absolute hell, but they'll be well-equipped for dealing with the adult world.
Yes no-one reads the ToS but by the same token we all know we take a risk when doing so. Google did nothing wrong in this instance, they just held the person to an entirely reasonable and fair condition of use.
That's like a criminal blaming a witness for them getting caught.
If the lie wasn't there nothing would have happened, you have to look at the root event, not what followed.
That inability to form contracts below a certain age actually protects children and is in their benefit, no-one should want it removed. Yes there are situations where it seems overkill but on balance it's a good thing.
So given that the law is reasonable it has not, as with most things with kids, fall back on the parents.
(Note: My understanding of the law is based on English law rather the US law but I'm guessing that something similar applies)