Can the Gospel be replaced by a children's pageant on Christmas?
No. The Church accords “special marks of honor” to the proclamation of the Gospel in the liturgy, as it’s the high point of the liturgy. Omitting the reading of the Gospel or replacing it with a skit or some other kind of dramatization is not permitted. This is clearly stated in the Directory for Masses with Children.
Directory for Masses with Children (Sacred Congregation for Divine Worship):
41. Since readings taken from holy Scripture "form the main part of the liturgy of the word," even in Masses celebrated with children biblical reading should never be omitted.
47. When the text of the readings lends itself to this, it may be helpful to have the children read it with parts distributed among them, as is provided for the reading of the Lord's passion during Holy Week.
48. The homily explaining the word of God should be given great prominence in all Masses with children. Sometimes the homily intended for children should become a dialogue with them, unless it is preferred that they should listen in silence.
One option for the reading of the Gospel in parts is during the Passion narrative on Palm Sunday and Good Friday. It should also be noted that the rubrics for this traditional passion narrative does not allow for dramatization (performance).
33. The passion narrative occupies a special place. It should be sung or read in the traditional way, that is, by three persons who take the parts of Christ, the narrator and the people. The passion is proclaimed by deacons or priests, or by lay readers. In the latter case, the part of Christ should be reserved to the priest.