When may the Eucharistic Prayer for Masses with Children be used? Is it the decision of the priest celebrating the mass or are there guidelines for this?
Concerning Eucharistic Prayers for Masses with Children, I would refer you to number 4 of the 1974 decree of the Sacred Congregation for Divine Worship, Postquam de Precibus:
"Use of a Eucharistic Prayer for Masses with Children is restricted to Masses that are celebrated with children alone or Masses at which the majority of the participants are children.
"A community of children means one so considered by the Directory for Masses with Children, that is, one consisting of children who have not yet reached the age referred to as preadolescence."