I started one 18 months ago with about the same number, for 6-9 yrs old. We do 40 mins singing, then a game, usually outdoors, then about 30 mins Eucharistic adoration. We don't sing a mass, but our new family choir (adults and children) has many of our schola members in it, so we have some overlap and can use some of the same music.
I decided that over four years they would learn to read chant (mostly using ward methods, they do this in the first year), that they would be able to sing all the chants in 1974 Jubilate Deo book in both Latin and English versions, and we also learn a canon or round every week, as a beginning to learning to sing in parts.
It is also a useful forum for catechesis as we go - saints days (each term is a different saint), what is intercessory prayer, singing and praying at the same time, etc etc. The boys learn to set up the altar (the girls let them have this job in case they will become priests) the girls light Lady candles and pray at the altar of our lady. (just so everyone gets to light a candle somewhere).
We also have gone to a pueri cantores day, very useful for them to hear more advanced choirs.
Ward has plenty of practical suggestions, keep working on tone and pitch, move quickly from item to item so as not to cause fatigue, etc. Her boos are well worth a read.
Children have their own folders which they colour to make their music as beautifu as the monks (we looked at the Book of Kells last year).
Can't think of anything else to say. Oh we have a schola party at the end of every term - Benediction with smells and bells, our best music, followed by goodies and games. We give out invites for them to bring friends and family, a useful evangelising / recruiting tool.