SHTF and you're really going to be concerned about whether your diapers are officially "nappy grade"? Fold up and pin an old t-shirt on the kid and call it good.

Traditionally, old rags were used. I'm sure we'll have plenty of old rags as our walmart t-shirts start to fall apart, since that takes all of 30 seconds.

Also, our kids are in diapers waaaaay longer than they ever have been. Staying in diapers until 3 is really not normal or ideal. Read about elimination communication. I know from experience that it works. Kind of a pain, and most in our current society aren't going to be bothered to do it with the ease of disposable diapers or machine laundering cloth, but it's something to keep in mind.