Kids can get government IDs. In my state, it's free for anyone under the age of 17. It's also free for anyone who can provide adequate proof they can't afford the $15.
A birth certificate is $25. My state has a programme for people who can't afford that to get a grant to pay for that, too.
I used to be quite anti-"comrade, your ID, please", but now that it's required anyway to function, we might as well get some benefits from it. Such as attestation of phone numbers being "this is from a real person who is an ordinary consumer".
Getting a certified copy of a birth certificate requires proof of identity. If you don't have a government issued photo ID states typically require multiple forms of secondary documentation (some of which themselves require photo ID to obtain). The secondary documentation can be expensive to obtain (fees for the documents themselves plus the effort to locate the documents).
The secondary documentation that can be easily and cheaply obtained without a photo ID, such as a copy of a utility bill, can fail because of name match problems--if your birth certificate is for "Bartholomew Jo-Jo Simpson" and your electricity bill is for "Bart Simpson" that might not be accepted.