My guess is that kids of this age would enjoy TRIBBLES most, for obvious reasons. Maybe even I, MUDD for similar reasons.
I'm not sure what they'd think of METAMORPHOSIS or METHUSELAH.
I was about to say MIRI and certain other episodes might be too intense for the average five-year-old, but as long as you can wash it off.....
Still, are there classic episodes you might be hestitant to show to a child? CHARLIE X, THE OMEGA GLORY or BY ANY OTHER NAME, for instance? I'm guessing Kelso's daughter would go to town on CHARLIE X. (''EWWW!!!! An old person!!'' and ''Where's her nose?'' and such.)