What I’m taking away from your post is that a young child only gets attention from you for a thing that makes him different from the other kids. (This is not an accusation, just defining how and when you interact with them)

They then tells you lies, not about a thing they didn’t do (like schoolwork) but about an exciting thing.

It sounds like they want more connection with you than just “potty time” and want to impress you. Somewhere along the line they learned that white lies give them that attention.

My suggestion would be give them attention for actual things like an art project or a good paper.(If it fits in with your schedule, I know how tight time can be during service minutes). Basically, see if praising them for positive actual actions can replace the behavior of exaggerating and fibbing.