What you are trying to provide is the taste of victory. The child who wins occasionally with an adult will go on to tackle other problems, like expressing feelings, or doing homework.
Of course no one wins all the time and you only succeed if you try. Through roughhousing you can teach a child how to lose, too. Sometimes you win in life and sometimes you lose. The point is to do your best, to keep trying, and to improve your techniques.
"This is the way the farmer rides: Clipety clop, clipety clop.
This is the way the ladies ride: Tritree, tritree, tritree.
This is the way the gentleman rides: Gallipe, gallipe, gallipe."
Say aloud as you suddenly spread your knees:
"Trot trot to Boston
Trot trot to Lynn.
Watch out (child's name),
Or you'll fall in."
"Papa went a hunting.
Papa shot a bear.
Where did he shoot him?
Right in there!"
For variation, tell your child that when an adult presses his or her belly button, the adult becomes no longer ticklish. The child has to repress the adult's belly button to make the adult ticklish again.