You must invent a story for them.

I have thought of cholera, but that is played out; you know we have already tried it on The Boy who was Thrown Away.

Invent something quiet, peaceable and respectable--as far removed from fighting as possible.

What do you say to measles ?" "Not half bad," I returned.
"Measles let it be, then! Say I caught it from Wee Willie Winkie.

You do not think it too incredible ?" he added timidly.
"Not more than YOUR story," I said.
He grasped my hand, struggling violently with his emotion.