sorry it has taken me a day to clear up the practice. what is happening is that 3 children come up to the ambo and read the whole “you have come to save sinners, lord have mercy, etc.” part. I was under the understanding that this is the duty of a priest, deacon, or as is the case on most sundays, the music minister. if anyone knows differently, please let me know…
If they are using the ambo this is a liturgical abuse, failing to following 2002 GIRM n. 309:
“… From the ambo only the readings, the responsorial Psalm, and the Easter Proclamation (Exsultet) are to be proclaimed; it may be used also for giving the homily and for announcing the intentions of the Prayer of the Faithful. …”
It could be argued that the Kyrie is meant to be sung, from n. 52: “When the Kyrie is sung as a part of the Act of Penitence, a trope may precede each acclamation.” However the GIRM gives considerable flexibility in n. 40: “Although it is not always necessary (e.g., in weekday Masses) to sing all the texts that are of themselves meant to be sung, every care should be taken that singing by the ministers and the people is not absent in celebrations that occur on Sundays and on holy days of obligation.”
But I think the choice of suitable ministers depends on the parish priest. From 2002 GIRM n. 107:
“107. The liturgical duties that are not proper to the priest or the deacon and are listed above (cf. nos. 100-106) may also be entrusted by a liturgical blessing or a temporary deputation to suitable lay persons chosen by the pastor or rector of the church.”
The 1973 Directory for Masses with Children includes:
“Offices and Ministries in the Celebration
22. The principle of active and conscious participation are in a sense even more significant for Masses celebrated with children. Every effort should therefore be made to increase this participation and to make it more intense. For this reason as many children as possible should have special parts in the celebration: for example, preparing the place and the altar (see no. 29), acting as cantor (see no. 24), singing in a choir, playing musical instruments …”.
(This is from Chapter 3 “Masses with Children in Which Only a Few Adults Participate”. This is unlikely to be the case at a Frist Communion, but I can see how a priest could want to follow this approach for it.)
Clearly a child could be cantor and sing the Kyrie (from a place other than the ambo).