There is no guarantee that you will be able to have a record expunged, even if it falls under a category that permits expungement, (guilty pleas or convictions of felonies and misdemeanors in PA are not eligible for expungement). You should contact an attorney in PA to provide more detailed information.
I saw in your answer to Mr. Shields question that you have not yet either appeared in court or admitted guilt. Rather than worrying about expungement at this point, you should contact an attorney to defend you against these charges. This way you will be in a better position to avoid conviction in the first place.
Having come late to this party and having the benefit of the greater detail developed in the back and forth that has taken place, I wish to try to avoid possible confusion based on answers that have come in as the detail available has increased. First thing you must do is make sure that you guys have had your pleas of guilty entered and requests for a hearing filed. I practice in the western part of the state where procedures may be different than in Allentown. Because Retail Theft is graded (the seriousness of the charge ranging from a summary offense to a Felony 3) in part based upon the existence and number of prior offenses, all such charges require fingerprinting so the Commonwealth can keep track of the number of priors. Accordingly in my part of the world, unless prior(s) are known at the time of police involvement, the case is sent out as a Non-Traffic Citation charging the offense at it lowest grade possible. A Fingerprint Order also is sent requiring that fingerprinting be done. When the results of that procedure are received the Magistrate's Office then knows whether a higher grading is appropriate because of one or more prior offenses. Regardless of what impact if any the fingerprint results have, a Criminal Summons is sent out with a date and time for a hearing. Since I do not practice in Lehigh County, I suggest you talk with (I think in person is least subject to confusion) the personnel in that office about what you can expect as far as procedures and what steps you and your girlfriend must take to plead not guilty and have a hearing. An attorney who practices regularly in your county would make sense. Most here offer free, if limited, consultations. Take advantage of that to talk privately with one or more in your area. Get yourself an education and hopefully find a lawyer that you want to have help you out. Fees can vary considerably so be a smart consumer and shop around. Good luck.