"we sing silly songs" is a great tape for kids and has a few nice rounds. My fave, one bottle o' pop, looks odd on paper but sounds really good when sung right. It is sung in three parts.

One bottle o pop
Two bottle o pop
Three bottle o pop
Four bottle o pop
Five bottle o pop
Six bottle o pop
Seven bottle o pop
Pop

Dont put your muck in My dust bin
My dust bin
My dust bin
Dont put your muck in My dust bin
My dust bin's full

Fish and chips and vinegar
Vinegar
Vinegar
Fish and chips and vinegar
Vinegar and pop