How would you explain this science activity to young children?

-I found these cute valentine balloon things called wack-a-pack. each self inflating balloon contains distilled water, citric acid(vitamin C) and sodium bicarbonate (baking soda). Instructions are to smack the pack and shake, then quickly throw on the floor. I need a good easy way to explain this to ages 4-7.you could try mixing the chemicals where they can see what is happening

and then explain how inside the pack the same chemicals are there

then blow up the wack a pack thing.its hard to figure out how to explain it based on your description, sorry.