Don't do it! Or do what Jack wisely says and treat them like any other client--ask the due diligence questions and charge your normal fee.
The problem with not signing as a paid preparer is all the fraudulent preparers who do that and get caught. We've all reviewed many EITC returns from prior years that say "self-prepared." Yea, right. The IRS notices too. Of course if you do this as a freebie you can't sign as a paid preparer. But the bad preparers didn't sign and did get paid so you'll get lumped right in there with the rest of them. Do you really want a call or letter or visit from the Office of Professional Responsibility?
Explain to the parents that you'd really like to help their children out but the IRS has strict controls over EITC returns and the responsibility is so great that you can't do it without interviewing the children and charging a reasonable fee for the risk. I too would refer them to Block or JH--just tell the parents these chains are experts in EITC, which they are, and can do a much better job than you because they are so well versed in the EITC rules.
In our office we do very few EITC returns, all for clients we've known for a long time who own businesses that had a very bad year, just got divorced, whatever reason for their income to plummet. We don't take many new clients, and those we do who turn out to be EITC-eligible get referred right out the door. Hard to do sometimes when they were sent by a good client, but it's just not worth the risk.