Cases involving motor vehicle are of 2 types. One is before the Motor Accidents claims tribunal and the other u/s 279 of I.P.C. for rash and negligent driving.. It seems your case is u/ Motor accidents claims tribunal.. You state that the Petitioner has admitted his negligence. You further state that you have FIR etc to prove the negligence of the driver.. Your query is whether admission by the petitiioner of his negligence witll be damaging to your case. Relevant judgment in your case is
 Damaging statements cannot be got rid off merely by stating that they were a slip of the tongue. [ AIR 1982 Calcutta 488 para 11]
 Section 31 of the Indian Evidence Act states " Admission not conclusive proof but may estop. : Admissions are not conclusive proof of the matters admitted , but they may operate as estoppels .
 The admission by a party must be read alongwith the evidence given by him in his examination in chief examination [ AIR 1971 SC 1542]
In your case the admission cannot be read in isolation but has to be considered alongwith all the ohrer evidence. Try damage limiting exercise by thoroughly cross examining other witnesses. Motor accident claims tribunal is a tribunal and strict rules of evidence are not applicable to it. . Further the Supreme court has held that motor vehicles are death traps . The degree of caution to be exercised by the driver of the vehicle is definitely more than that of persons and especially so when the petitioner is a small child, women or old persons [ in whom the reflexes are slow.
If the Ld Judge if at all considers it to be a case of contributory negligence then you can enhance the claim amount by using latest judgments wherien maximum amount is awarded in cases of injury suffered by your clients. For eg. if a hand cart puller sustains fracture of both his legs or if the thumb of a typist is lost the disability is 100 %.Try to obtain judgment in the case involving the driver filed against him u/s 279 IPC and if it is in your favour use it. Dont worry, hard work pays.
I LIKE THE ZEAL WITH WHICH YOU ARE WORKING.