Hi Anna,
It sounds like you’re asking how to composite your kids onto the surface of the wavy CD. That’s an art as well as a science. If you Google “photoshop” and “compositing” you should find several good books and articles on the topic. Also, visit https://www.planetphotoshop.com/tutorials_effects.html. Study the tutorial titled Combining Images Using Masks. That’s how I usually composite photos. I apologize in advance because what you may find may seem overly complex for new or casual users of Photoshop. Successful compositing is a complex topic though. I had started writing some long-winded tips, but thought this might serve as a better intro to the topic.
In short, basic compositing will involve these skills: selecting specific areas of a picture, masking that area, moving the masked layer from one PS file to another and arranging the layers in proper order, resizing layers as needed, adjusting color, brightness, and saturation to fool the eye into thinking the objects belong together, and blending edges where the layers meet.
I hope this is helpful.