Symbolism, Imagery, Allegory
Bet you saw this one coming. Hieroglyphics are literally symbols, after all.
When Toby and Ken join the Egypt Game, they buckle down right away to finish the hieroglyphic system. And no, not just to pass the time or get in good with the original Egypters. There's a bonus to having a secret language.
Basically, the hieroglyphics allow the members of the Egypt Game to communicate freely without worrying that other people will intercept their notes. Even when the hieroglyphics get loose at school, they don't have to cover their tracks:
They could write about things like when to meet in Egypt again, and what they thought of the teacher, and all sorts of other private information. Then if the messages fell into enemy hands, no one would know what it was all about. (14.8)
The hieroglyphics cement their group as a little secret society, and also allow them to communicate in a different way. They let them talk about Egypt outside of Egypt, so that they can bring it into every part of their lives instead of keeping it all trapped in the abandoned yard. Plus, it's a whole new way to pass notes without getting caught. Isn't that every kid's dream?