Not a big problem to get 6 mon

For most B2, when entering the States, they will be granted an I94 valid for 6 months, no matter what, even their visas have annotations like your parents.

So just show the officer the roundtrip tickets. My parents did that, and their I94 is valid till the day they are leaving, which is exactly 6 months + 1 day. They arrived on May 21, their returning date is 11/21. So they were granted an I94 valid through 11/21/05.

The only drawback to have such an annotation is, next time when your parents go to apply for new visas, their length of stay will be especially inspected by the visa officer. He will be really interested if they obey it or not. If they fail to do so, they have a greater risk to be denied a visa again.

But yeah, one month is too short. Just let your parents stay as long as you want them to.