This.
The kids are adamant? Tell them they will sing in the chorus and LIKE IT and otherwise STFU! I’ll sure the priest will be delighted to lay down the law to them. 
Sooooooooo, what’s the problem with giving all the kids more lines?
Why not cast kids in the adult roles and adults in the kids roles? Use a little creativity!
You might find it to be much more entertaining for the audience too.
A serial killer with birth control glasses, evidently!