Okay, are you saying that for a period of time before the child support order was set in place the person was in prison? I definitely would check your laws to see A. If there is a statute of limitations on child support and B. if so how would they calculate that retro period.
Keep in mind that retroactive child support (i.e. child support set for a time before the obligor knew about the obligation) is different than a modification in circumstances (such as incarceration) that would affect your ability to pay current child support. Check with your lawyer.