What do these words mean? Ego sum custos engraved on a plaque?

A small molded wall plaque shaped like a church window - could be ivory, but I'm pretty sure it is plastic. It depicts Christ with his arms extended. To His one side is a mother with son and to the right is a father and daughter - the parents are facing Christ and kneeling (I am uming the relationships). The words "Ego sum custos" are imprinted below the scene. Above Christ's head are the initials IHS (Jesus in Greek) .(The 1st letter could be an E or C, or maybe even Ergo(?) - actually looks like 2 letters together.