No more onInvalidity event for ChildrenAgeInput

It appears that with native type="number" inputs, when you've entered a number,
and then typed some random alpha-chars on the keyboard, they aren't picked by
the input. However, those alpha-chars do make it to the onChange, and trigger
invalidity. Thus, the invalid state persists, even though the native number input
doesn't display the just entered invalid (alpha) value.

Therefore, a good solution seems to be ignoring the invalid chars after
something valid has been entered. Sorry, IE8 users.