For kids exams are probably just compiled stress, bu if your old enough to really appreciate good criticism, then they can be really helpful.
First off, never strive to do good in exam to have bragging rights. Im sure you already know this but its important to not get in over your head because youll start to play for the wrong reasons and will stunt your potential as a musician.
100$ is a bit steep, and if money is a problem, maybe its not that worth it, but if you look at it in the right light, that 100$ also makes you extremely motivated to do well and you will find yourself practicing more frequently and thoroughly.
Being judged isnt something anyone likes to do, but in this case you are being judged by people who (if there credentials hold up) could really have important things to say. Remember examiners arent mean people, they understand the hard work you put into music and only want to see you progress; they are your friends. (This isnt some serious piano competition)
I like the idea of modeling for you kid, I can see how that would be motivating.
Anyways, the decision is up to you and even if you say decide no, theres going to be exams in the future to give yourself more time to physch yourself up. Dont sweat on it too much, just enjoy playing and push yourself harder to practice and youll be doing the right thing