1. Gmail will include the "secret" account address with every email they send (in the "Sender" header and in the envelope-from address ("Retunt-path"). It will be no safer from spammers than the other address.
2. A brand new email address that is not too short or a very common name used by a child will not receive any spam. It's not a new account of an adult that would be fed into every online merchant's site and immediately shared with "select partners" a.k.a. spammers (I was going to write "an account you setup for your wife" but decided to avoid male chauvenism). Though I might be completely wrong about what children do with their email addresses. My kids only exchange emails with very few friends and teachers.
3. Google's spam filtering is not anything close to being "good". They not only miss a few. They also have a quite high rate of false positives ( I saw more than 1% one month that I made counts). They provide absolutely no control on how spam is filtered (such as sensitivity or opting out of spam filtering) and they do not pass detected spam through filtering rules. There is no way to define rules that precede spam filtering. Also there have been reports on legitimate mail that Gmail haven't even filed in the spam folder (or whatever it is in Gmail that's "not a folder").
4. Anyway the spam is not the real problem since the child is not going to get any. My two sons have email accounts for about 3 years and not a single spam message (but they don't use them to "sign up" for anything, and if they do need to sign up I do it for them with disposable addresses).
5. The real issue is that parents have a esponsibility to watch their childrens actions.
Recently my 8 years old son was required to provide an email address "of his own" to his teacher to communicate with his classmates. What I did is provide an alias in my domain and create a rule to forward a copy to his mailbox and keep a copy in my inbox. I also created a personality in his account that uses he address in my domain by default and bcc's by account with all his outgoing mail (all this is using fastmail.fm that hosts my domain so there is no revealing of his account's direct address). That way I can monitor all his mail. Usually I get his mail before he does because I watch my mail much more often (Actually I setup email notification on his computer so he gets notified in real time about incoming email but he is not online all that much). What I plan nest is to open a family account at fastmail.fm and then I can (from my account that I will setup with admin privileges) watch the childrens accounts (when they grow older I can use the "privacy" option that allows the admin to be blocked from reading content of individual accounts so older kids can have privacy but still have dad pay for their account).
BTW: I don't have anything to do with FastMail.fm except for being a happy customer for many years.