Since Pop Rocks have a miniature explosive effect once they come into contact with moisture, you can easily make a play on words regarding this explosion. You can design the valentine to look like a ticking bomb, a piece of dynamite or a drawing of an explosion. On top of the picture, write something like, "Our friendship is explosive!" or "Valentine, you're the bomb!"
The term "Pop Rocks" is also open to a play on words relating to being a rock or pop star, or using "rock" as a synonym for "cool." On the outside of this valentine, you can have a picture of your child's or their friend's favorite pop star, a microphone or an instrument like a guitar or a drum. Above the picture, write, "Valentine, you rock!" If the recipient loves to sing or entertain, write, "Happy Valentine's Day to my favorite pop star!"
Valentines with Pop Rocks also work well for children who like science. Because the effect of the Pop Rocks is a basic chemical reaction, you can have a great play on words here as well. Your child can give a valentine with Pop Rocks inside to their best friend or their boyfriend or girlfriend with the message, "Valentine, our chemistry is explosive!" or "Our chemistry rocks!" On the outside of the valentine card, you can draw a chemistry beaker with a chemical in it, or you can depict a chemical reaction between two chemicals mixed together.
Since Pop Rocks create a tingling sensation on your tongue and on the sides of your cheeks once they're inside of your mouth, you can use a play on words about this aspect of the Pop Rocks to create a cute Valentine's Day message. On the outside, draw someone with bumps or someone that seems to be shivering as a way to represent internal tingling. On the inside, use the message, "Valentine, our friendship makes me tingle!" or "You make me feel all warm and tingly inside!"
On the inside of the card, use tape to adhere a small package of Pop Rocks candy and have the giver of the valentine sign his or her name. This doesn't mean that you can't get creative and add a small picture or another message on the inside, but since you and your child have already put considerable effort into the outside of the cards, the inside can remain simple without taking away from the meaning of the card.
Another option for the inside of the card is to move the cutesy play-on-words saying from the outside of the card to the inside and leave the picture to stand alone on the outside.