Chapter III, Part 4. Singing and Music
30. Singing must be given great importance in all celebrations, but it is to be especially encouraged in every way for Masses celebrated with children, in view of their special affinity for music.  The culture of various peoples and the capabilities of the children present should be taken into account.
If possible, the acclamations should be sung by the children rather than recited, especially the acclamations that form part of the eucharistic prayer.
31. To facilitate the children's participation in singing the Gloria, Credo, Sanctus, and Agnus Dei, it is permissible to use with the melodies appropriate vernacular texts, accepted by competent authority, even if these do not correspond exactly to the liturgical texts. 
32. The use of "musical instruments can add a great deal" in Masses with children, especially if they are played by the children themselves.  The playing of instruments will help sustain the singing or to encourage the reflection of the children; sometimes in their own fashion instruments express festive joy and the praise of God.
Care should always be taken, however, that the musical accompaniment does not overpower the singing or become a distraction rather than a help to the children. Music should correspond to the purpose intended for the different periods at which it is played during the Mass.
With these precautions and with due and special discretion, recorded music may also be used in Masses with children, in accord with norms established by the conferences of bishops.