Our children's choir sang the Kyrie of the Byrd 3 part yesterday (With help from a couple of adults and some older teenagers.)

First the Marian anthems
Secondly the Ordinary, Asperges, Mass I, IX, XI, and XVII and Credo I

Our children's choir are singing the psalm verses for the Introit, they seem to work. For the easier Introits they are also singing the chant.

Office Hymns are another option,
O Salutaris (sung to the Office melody of the day / season)
Jesu Dulcis Memoria with extra verses taken from the Matins and Lauds Hymn.

Or chants with a chorus
Veni veni Emmanuel
Puer Natus in Bethlehem
Rorate Caeli de super
Attende Domine
O filli et filliae

Will ask, I don't run the children's choir.

To tomjaw’s suggestions, I think that Salve, mater misericordiae is kind of obligatory in trad land especially. It is not my favorite, but it is handy. Salve, festa dies is one of my favorites but is, on the other hand, used once a year and is tricky.

Anyway, the simple repeated chants like Parce, Domine and the Cor Jesu sacratissimum seem necessary especially if you have the kids sing at Benediction; the Adoremus in aeternum (the mode V from the Solesmes/Vatican editions is nice; we switch to one in mode VI, which I am fairly sure is from the Westminster Hymnal in Advent and in Lent) is also nice. And then you have the O Salutaris and Tantum; around here, we are lucky that there is adoration almost every day from Tuesday through Saturday and even on Sunday when the university is in session. Most of those places use a metrical tune for the O Salutaris but then use the mode III Tantum (this has some issues; people come in randomly and often lob off the first note of the first podatus of “veneremur”, but it’s still a good problem to have).

I guess that’s more than five, but I know of a parish where they do benediction monthly after Mass. It’s not my thing to do it that way, but it’d be a place where you could regularly have the children leading the singing instead of adults. And that would be three to four pieces since in the trad world, you do an exposition chant (not necessarily O Salutaris! Could be Ave verum, Rorate caeli in Advent), a Marian chant (the seasonal antiphon etc., Inviolata, the Recordare offertory, the first form of the Litany in the Liber, the Sub Tim all being frequently used choices), something for the pope (Tu es Petrus from the office of Saints Peter and Paul is common), maybe the bishop (Oremus pro antistite for example), and then the Tantum plus a reposition chant. Lots of these are as MJO says in the Liber Cantualis. My requirement is typically that it be in a Solesmes edition, and my preference is that it be accompanied, so if Potiron or someone else wrote one, great. If not, I have to improvise. (There are a lot of O Salutaris and Tantum melodies in Cantus Selecti, some of which are just not that great, with all due respect given to Dom Pothier and to his successors.)

And thinking further: maybe you teach the kids the Tantum, but not the corresponding O Salutaris. It’s generally got a harder melody!

After the Mass Ordinary (and perhaps Marian antiphons), I teach rounds: Jubilate Deo (Praetorius), Alleluia (Boyce), maybe Dona Nobis Pacem. I also do the Dubois Adoramus te, Christe. Great for working on solfege down from high Do.