Flying with kids- how young is too young?

I have a friend who really wants me to take he & his two boys up- they
are 3 & 5. I am worried about them "freaking out"- especially the
little one. They have never flown before (commercially or otherwise).

Of course they need to be in back of the 172, which would make it
difficult for my friend to reassure them, etc.

I'm especially concerned about the younger one for the obvious reason
that 3 is pretty young. To the folks that have a lot of experience
taking up little kids- what are the rough odds that he'll be just fine
vs. having a cow? I need to decide if I should suggest that this isn't
a great idea or not. TIA.