KeyboardLock | Just do | Keyboard library

Replace Destroy() with DestroyImmediate() in your script.

Here is a detailed explanation:
If you use the Destroy() function to destroy a GameObject, Unity doesn't destroy it right away. What it actually does is just mark it as null and destroy it in later frames. So if you destroy a GameObject in one frame, and query for the child count in exact the same frame, the number you get will not be decremented. If you do it with DestroyImmediate(), the GameObject, as it is marked in the function signature, will be immediately destroyed, and the child count will be decremented instantaneously.
In your case, the CreateWords() function destroys the created letter object if there is already another letter same as the one you've created, and then call the function itself again. However, even if the letter object is destroyed, the child count still increases by 1. At last, the function will find that every panel has more than 6 letters (while actually there aren't), causing an infinite recursive loop which crash the program.