Would a prior assault felony from 20 years ago prevent seeking child custody from philadelphia department of human services

This is a conundrum. Because of the record the agency would normally be justified in withholding custody. But normally courts and agencies are expected to disregard convictions that are more than 10 years old. this is particularly so when there has been no further, more recent, convictions.

You have two avenues to proceed. The first is to take an appeal of the agency's decision. You could, and probably should, seek to obtain a pardon and expungement. When a person has paid the debt and demonstrated to be an upstanding citizen and when that prior conviction poses a burden then the governor can pardon a person for the crime. After a pardon is received then a party can seek an expungement of the record. In that case the person can legitimately answer that their record is clean and the agency MUST ignore any information to the contrary.

Again, you should engage an attorney to appeal the agency's decision.

Regards,

Roger