Geez, VAC, let’s enjoy Christmas for its full 20 days. But, as someone who has planned liturgies, I hear you.
Okay, this is what our children’s choir at the Cathedral used to sing:
Attende Domine (get the English translation from the Worship III hymnal - the older one, not the newer one, as it’s closer to the original)
O Sun of Justice
The Glory of these Forty Days
Our Father, We Have Wandered
Again, We Keep This Solemn Fast
There’s a Wideness in God’s Mercy (Babiloine melody)
Tis Good Lord, to be here (2nd Sunday of Lent)
I Heard the Voice of Jesus Say (Third and Fifth Sundays of Lent)
Your Hand, O Lord, in Days of Old (Fourth Sunday of Lent)
What Wondrous Love is This
I know that you said that Holy Week is pretty much written in stone, but, here are some suggestions that work well with kids (as we tried them with success):
All Glory, Laud and Honor
At the Name of Jesus (found in the Red Worship III hymnal)
At That First Eucharist
At the Cross, Her Station Keeping
Lift, High the Cross
Sing My Tongue, the Song of Triumph
You might also try an antiphonal entrance such as “Come, let us return to the Lord, for he is gracious and merciful.” It’s a setting from the Benedictines and the verse changes with every week of Lent. It’s long enough to cover the entrance procession to the time the celebrant gets to the chair (if he’s using incense, you can add a second verse).
I hope this helps.