Now that I have a 3-year old in the trailer behind me, we have a new hill/incline/gentle slope song. It's originally from a preschool tv show and goes:

"I can do it (climb this hill)
I can do it (climb this hill)
I can do it (climb this hill)
You can do it too!
Wiggle your nose (wiggle wiggle wiggle!)
Touch your toes
Count to three (ONE TWO THREE!)
C'mon everybody join in with me 'cause
I can do it..." etc etc

Fortunately we only ride short hills.