Come to find out, many choir pieces can be modified to feature the children on their own parts. As long as there are sections that can be sung in the children's Treble range in unison or 2-Part, and the words aren't too difficult, why not? Some children are eager to learn to sing parts, as well.
Use this as an opportunity to encourage life-long choir singers.
Rather than just absorb the young children to sing Soprano or Alto with their mothers, I made sure the unique children's sound was featured prominently on their own part(s). (I usually had the luxury of an assistant who could help cue the children and keep them focused.) The kids seemed to like it. We tended to use less complicated music and have shorter rehearsals. It was a win-win for everybody.
Here is an example of an easy 2-Part Mixed Chorus arrangement that could be subdivided or combined in many different ways to feature different sections, soloists, or even children. And the words are not very hard. "Count Your Blessings" 2-Part Mixed.
Other titles may be found at Music for Home and Family/Family Choir Arrangements