"You can tickle her feet later."

There's a time and a place for everything and now's not the time nor the place. Get yourselves back to her house, wait until she takes off her shoes, then ask her politely if she would like her feet tickled. If she gives the go ahead, then you can work your magic and get her laughing. But right now you need to load up in the car and get buckled 'cause we've gotta go. But then again I guess when you're four anytime is a good time to tickle feet, huh?