Whether you're dealing with adults or children, the first step in helping someone move on and adopt a cooperative mindset is listening to them until they feel good and listened to.
Small children live in the present moment, in the now. They are in deep, laser focused, inextricably tangled up in their object of engagement. Don't let that little one bogart all the wonder - take a toke next time it passes around. Join that magical mystery tour and strive to see the world purely, as through a child's, once more.