Yes, file the petition.  If you are not enrolled to practice in Tax Court, your clients will have to sign at as pro per (unrepresented).  Make sure it is mailed using one of the accepted delivery methods.  Post Office with delivery confirmation is good enough; there are some of the private delivery services (FedEx and UPS) that do not qualify for all of their options, usually the faster ones, because they are not on the list that was made up before they were offered.  Date of mailing is what counts, not date of delivery. 

Most petitions shouldn't run more than three or four pages, and shouldn't attach evidence or documents.  They should attach a copy of the Notice of Deficiency, with SSN's marked out.  Try to follow the rules on filing a separate form with SSN's, and a separate form designating where you want the trial.  (They're being conducted online for now, and most cases settle before trial.)  Also, send the $60 filing fee.  

An IRS attorney will file a generic answer but the case will be assigned to an Appeals officer, who may get around to contacting you in six months or so.  It's another six months before a trial date is set.  If you haven't settled with the Appeals officer by then, the IRS attorney will get involved. 

Check on whether your clients qualify for help from a low-income taxpayer clinic, if one is nearby.