An adaptation of the Catechism for Young Children in modern English.
Question: What did God the Father agree to do in the covenant of grace?
Answer: To justify and sanctify those Christ would die for.
Question: What did God the Father undertake in the covenant of grace?
Answer: To justify and sanctify those for whom Christ should die.
See question 45 for adapting “undertake”. Additionally, the answer is simplified from “for whom … should die” to “… would die for.” Modern audiences (and certainly children) don’t care if your sentences end in prepositions.