Depends if you wanta keep 'em busy or send 'em to the Sandman ;-)-
For entertaining them, I would recommend to one and all the songs of Barry Louis Polisar - wonderful, funny, some sweet but non-smarmy. Stuff kids can relate to, like "Stanley Stole My Shoelace and Rubbed it in His Armpit" and "I Don't Wanna Go to School (Tom said to his mommy)" - wherein Tom turns out to be the principal. One I've used with preschoolers that NEVER FAILS to get a great response is "Don't Stick Your Finger Up Your Nose." lol
There are lots of little songs and chants you can turn into action songs - Dance to Your Daddy, Five Little Monkeys Jumping on the Bed, etc.
Or, with a year-to-two year old, a good knee-bouncing song is Pony Boy/Girl.
For getting 'em to sleep, lotsa good, repetitious, rhythmic stuff that goes well with rocking - Froggie Went A-Courtin', Stewball, Puff, Come by the Hills, Waltzing with Bears (obviously this has been mentioned before - just what is wrong with it, Art?). Rhythmic popular songs work well too - I remember my father singing "Gonna dance with the dolly with a hole in her stockin,' dance by the light of the moon" while rocking me. Even stuff that is moderately more upbeat works in combo with the rocking - Tell Me Ma, I've a Laddie in Americay, Leatherwing Bat, etc.
And, you are EXACTLY right, regarding pre-teens. My eldest, who generally hates folk music, absolutely SWOONED over Little Musgrave and listened raptly - it was soooooooooooooooooooo roMANtic! *vbg*
Sinsull mentioned James James Morrison Morrison - love that poem! (Remember my mother reading it to me, lo these many moons ago.) Kids are great consumers of this kind of stuff too. Another that always goes over well is Maurice Sendak's "Pierre" ("There once was a boy named Pierre/who only would say, 'I don't care.'"). Carole King & Maurice Sendak also collaborated on a kids' musical - really more of a series of songs strung together, although there is a title character & all - and she came up with a nice melody line for his "Chicken Soup with Rice." The name of the musical is "Really Rosie," and the sheet music for it has been published - don't know if it is still in print.