Growing up I knew a very large family, seven children! I had already (before meeting them) had fallen in love with the idea of having a gaggle of kids. The idea of giving each child his or her own song however was directly stolen from them. Even though I will not be having seven, or even four children (my husband did not board my crazy train) I did decide to run with the song idea. Now 3 times I have poured over music until finding a tune for each of my children. Charlotte and Max love their songs. We play them now only a couple times a month, usually on a long car ride. About the time that they are getting really sleepy and about to drift to sleep, cozy in their car seats, one of them will ask for their song to be played. So this Mama rolls to it on her IPOD and while they listen, I get to remember picking it out for them and singing to them while they were inside the belly.
Willa's Song is Happiness by The Weepies. Was unable to find a decent youtube video of it. If you have the time and inclination, give it a google and a listen.