One thing I miss from my Protestant days (in addition to being able to sing Christmas songs during Advent when the rest of the world is singing them) is singing from hymnbooks in harmony. Harmony is reserved for the choir most of the time in Catholic churches I have seen.
For those who want to go beyond the standard, reserved singing-from-the-pews experience, I propose the Family Choir Section.
Once a month could be Family Choir Week and a poster would be hung in a part of the church near the organ (or whatever you may have) that says "Welcome to the Family Choir Section". Anyone would still be welcome to sit in that area, whether or not they chose to participate.
Songs for that day would be chosen to include some that are more participative. This could involve songs with hand motions/sign-language or harmonies. A quick practice could be held in that pew section ten minutes beforehand and someone could stand in front and lead the section during Mass.
This would take place during the regular music of the Mass. No special solos, just a little extra way to participate that day if people want. It wouldn't be expected to be great art. Kids of any age could try to join in.
And it would also be ok to NOT participate. But let the poster be your warning and watch out for stray, flying elbows on the hand motion songs.