Imagine you're watching a delightful movie on TV, and you get a phone call from your friend, who is watching a horror film on a different channel.
You don't mind that your friend likes scary movies, but you're clear that you want to stay focused on your happy story.
So you start talking about your movie, and after a while your friend realizes that s/he would rather be experiencing joy than fear, so s/he changes the channel to the same frequency you're tuned to.
Now you're both "on the same wavelength"... literally! That's the art of uplifting.
So when you're feeling good, and your child is upset, stay focused in your place of Well-Being... AND stay fully Present with your child.
Make peace with the fact that s/he's upset, and know that s/he'll be feeling good again soon. Before too long s/he'll choose to change channels from tragedy to comedy. :)