Let's look at your sentence: "Climbing holds are like rock edges and pockets you use to climb."
This sentence is almost correct but can be made clearer by adding a word to describe the 'edges' and to make it easier for a 6-year-old to understand. Here's a better way to say it:
"Climbing holds are like little rock edges and pockets that you use to climb."
This version adds the word "little" to show the holds are small, and "that" connects the ideas smoothly. It’s simple, clear, and grammatically correct for young children.