A Catholic cannot stand as a godparent at a non-Catholic baptism because a Catholic cannot promise to assist the parents in forming the child in a faith that is not his own. If the baptism is valid, the Catholic may attend the baptism as a Christian witness.
If the child’s parents make legal provision for you to be guardian in the event of their deaths knowing that you will raise the child Catholic, that is fine if the child is below the age of reason. If the child is above the age of reason, and thus able to discern whether or not he wants to continue in the Episcopal tradition in which he was raised by his parents, the priest will want to make certain that conversion to Catholicism is something the child desires. If the child wants to continue to go to an Episcopal church, you can continue to take him there (although you would be attending only in the capacity of an adult guardian accompanying an Episcopalian child). The usual conditions for Catholics attending non-Catholic churches, that you also meet the Sunday obligation to attend Mass and that you do not receive Episcopalian communion, also apply to you.