What is this "child pays for payment" thing
More intuitively, it could be called "receiver pays for sender." If a transaction lacks a transaction fee where it'd be able to be processed in a timely manner, the receiver ("child") can pay the tx fee so it'll be processed faster (or at all).
Does it mean that it would lead to another Tx in reality ? I mean to say, if Tx A has 0 mining fee and Tx B is using the o/p of Tx A, then Tx B can pay the miner fee for Tx A & Tx B to get them accepted fast ?