"Woops boom" is a declaration of pity to be used after something, like a pen or a child, takes a large fall. Often employed by mothers and babysitters. It is similar to oopsie.
Max fell off the slide.
Mom: Woops boom! Come here Maxie, I will give you a popsicle to make it better.
Gabi: Ah! I dropped my calculator and the batteries fell out!
Megan: Woops boom!