Let's say that, hypothetically speaking, your child comes home from Vacation Bible School with a note that says that he needs an outfit for the upcoming Toga Day, but you can't deal with it because you are driving directly from VBS to Cub Scout Twilight Camp, not that you own any plain sheets anyway. Well, fortunately for you, I have a solution for your oddly-specific problem!
All you need is a plain white adult-sized t-shirt, plus a cord that is a little longer than your child's waist.
Step 1: Have your child put his right arm up through the neck hole.
Step 2: Now his head goes through the neck hole.
Step 3: His left arm goes through the left sleeve.
Step 4: Tie the cord around his waist.
Finish the look with sandals.
Trevor wears a child small, so Steve's adult medium t-shirt was a perfect fit. A taller child would probably need a large or extra-large adult t-shirt.
By the way, this would make a pretty good Halloween costume for those of us who sometimes have warm weather on October 31. Or (again hypothetically) for anyone who has nearly-constant Halloween parties during the week before Halloween whose mother does not want him to ruin his costume before Halloween night. Just saying.