I'm surprised no one has brought it up yet, but your parents' visitor visas don't matter at this stage.
It's the I-94 stamp that matters. When they entered the US, how long were they allowed to stay? (The stamp on the I-94 would tell you that).... It doesn't matter if their visas exprire as long as their I-94 remains valid. Visas are for entry to the US, the I-94 is for the period of stay.
If by visa, you were referring to the I-94 the whole time, and it does in fact expire before you get the Notice for the I-485, then it's ok that they've overstayed for days, it will be forgiven as they are immediate relatives of a USC. And as long as their overstay is less than 180 days, they should be fine travelling with an AP while they await the GC. (Although the AP is always risky and doesn't guarantee reentry to the US).