The Eucharistic Prayers for children may only be used when a substantial majority of the congregation is children. e.g. Mass for the opening day of school. It would probably never apply on a regularly scheduled Sunday Mass. The priest is not at liberty to mix and match parts of different Eucharistic Prayers. The substitution of friends for disciples often relates to the question of the ordination of women. If Jesus was saying to his friends, “do this,” then the Eucharist can be confected by any friend of Jesus rather than only by someone in a constituted order.