The three-time-a-year tradition comes from the fact that there are three credit bureaus in the U.S., and each must give you one free check per year. It's just the maximum monitoring you can get for free. The different bureaus may have slightly different data, so it also has the advantage of making sure you aren't missing some error that only got picked up by one of them.
I don't know what events start a credit history. I suspect there may be some legal limitations to protect the privacy of people who don't use credit.
If a parent submits a name, date of birth, and social security number to check a free credit report, and there is no credit history yet, I really doubt the credit bureau would just discard the information that is submitted. Their purpose in life is to absorb and store data, and they're going to store everything they're legally entitled to. Just checking your credit history too many times affects the score, for instance, so that means they have to store a record of all inquiries. Also, all the credit bureaus try to use the opportunity to sell you their credit monitoring services, so they use your information for marketing if you don't opt out.
The question is whether whatever data they store in that case would be considered a "credit history" that would be entitled to legal protections like being able to get a fraud alert. The advantage of a fraud alert is that it prevents new credit cards and loans from ever being created without notifying you. It can be a pain if you're offered a big discount in a department store for applying for a credit card at the cash register, but it prevents someone from taking out a car loan in your name and driving away, leaving you with the car note.