Here are my three favorite books for rounds and choral warm ups:
This is the "round" book I like the most. I like it because it uses music through which you can teach good choral technique quickly to this age group.
This 2nd resource is one of my "go to" books when I am drawing a blank with warm up ideas on those days when I need to vary it up a bit:
This Rollo Dilworth book is my favorite though. It has so many fun ideas that teach great concepts. With this special age group, they learn best when it's fun yet meaningful! The ideas you find here fit that description.