My mom works with this person who wants me to give lessons to their third grader over the summer before they get real lessons in the fall. I'm pretty good at guitar, and have been playing for a long time, but I don't really know where to begin with teaching someone.

I also don't want to try to go too fast and him get frustrated and not be able to do something.

Should I teach him some easy open chords? Something else?
Make all of your exercises in the same key ( C would be logical ) so your students can learn the notes on their necks.

As for the exercices you can :
- teach notes
- make some easy chords progression
- teach chords construction and notations
- teach the major scale, and the pentatonic scale
- make some left hand position exercices
- make some right hand technique exercices (down picking, chord strumming, palm muting)

These are the basic exercices you can do with a beginner.
Hmm... Third grader.. That's pretty tough, personally I hate teaching kids (anyone under 7th grade)
Haha I just saw on wikipedia that 3rd grader correspond to 9 y.o children....

So a simple chords progression will be just fine.
I taught guitar to 7 year olds.... what hell that was.

"Yes, put your finger on the third space there, yes very - wait, no, what space is that?"
*kid stares at me*
"Do you want to learn to play a song?"
*kid stops staring and licks the guitar*


Anyway, on topic: start out slow, be extremely patient. Might want to start off with for instance Old McDonald or something...