The normal Gospel for Sunday 29 April 2007 was John 10:27-30, for Year C.
One justification for changing it would be the Directory for Masses with Children:
“43. If all the readings assigned to the day seem to be unsuited to the capacity of the children, it is permissible to choose readings or a reading either from the Lectionary for Mass or directly from the Bible, but taking into account the liturgical seasons.”
This is from Chapter Three “Masses with Children in Which Only a Few Adults Particpate.” But it has at the end of Chapter Two “Masses with Adults in Which Children Also Participate”:
“19. … Wherever the bishop permits, in addition to the adaptations already provided in the Order of Mass, one or other of the particular adaptations described later in the Directory may be employed in a Mass celebrated with adults in which children also particpate.”
Another justification is the 1969 Instruction
Actio pastoralis “on Masses with special groups”:
“6. … e. In the liturgy of the word, depending on the actual situation, texts may be chosen that are more suited to the particular celebration, provided they are chosen from the texts of an approved lectionary.”
(Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4, page 674).
Both these documents are endorsed by the 2002 General Introduction to the Roman Missal (GIRM):
“32. Special celebrations of Mass should observe the guidelines established for them: For Masses with special groups, cf. Sacred Congregation for Divine Worship, Instruction Actio pastoralis, on Masses with special groups, 15 May 1969: AAS 61 (1969), pp. 806-811; for Masses with children, cf. Sacred Congregation for Divine Worship, Directory for Masses with Children, 1 November 1973: AAS 66 (1974), pp. 30-46;”
Otherwise, even the bishop does not have the authority to change this reading. From the 2002 GIRM approved for the USA, which can be accessed from
romanrite.com/girm.html :
“374. In cases of serious need or pastoral advantage, at the direction of the diocesan Bishop or with his permission, an appropriate Mass may be celebrated on any day except solemnities,
the Sundays of Advent, Lent, and
Easter, days within the Octave of Easter, the Commemoration of All the Faithful Departed (All Souls’ Day), Ash Wednesday, and Holy Week.”