I suggest choosing some songs from the '60s/'70s/'80s or perhaps even older (e.g. pop standards, classical, Disney) as a way to expose them to something different from the current pop that they are exposed to in their everyday lives. That can be a lesson in itself, as a lot of kids are completely in the dark about music from the past.
I just made a CD for the kindergarten-aged daughter of some friends of ours. I love making mixed tapes/CD's for people and making them familiar with "old" stuff. Many years ago I made a cd of "love" songs for the son of some friends because that's what he wanted for his birthday. It must be 10 years later now, and they still listen to it

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Here are the songs I put on the most recent CD I made for the little girl:
Your Song - Elton John
At the Zoo - Simon & Garfunkel
Catch a Falling Star - Perry Como
Mango Tree- DoSul
Brand New Key - Melanie
Listen to the Land - Disney!!
Here Comes the Sun - Beatles
Up Up and Away - 5th Dimension
The Wind - Cat Stevens
Sing - The Carpenters
Good Morning Starshine - Oliver
I Don't Want to Live on the Moon - Ernie (the muppet)
I Say A Little Prayer - Dionne Warwick
The Rain Rain Rain Came Down Down Down - Disney
Rain on the Roof - Lovin' Spoonful
Rainbow Connection - The Muppets
Raindrops Keep Fallin' on my Head - BJ Thomas
Swinging on a Star - Bing Crosby
Windmills of Your Mind - Noel Harrison
Dancing in the Moonlight - King Harvest
Vincent - Don McLean
Silly Love Songs - Paul McCartney
Falling Through a Cloud - East Wind (This and the next are Pre-Illuminations music from Epcot)
30 Year Jig - Vasen