Ask them politely to give it back.

WWYDI your wardrobe led to another world (not Narnia or Middle Earth, something you've never heard of)?

Die of happiness and hope it's Evren (one of my worlds). 😛

WWYDI an Under-earth-man offered you a flab-cake?

Depends on how hungry I am..

WWYDI you found out the wardrobe actually led to Middle Earth?

Not mind much and go into it. I'd also start to wonder where all Prof. Kirke has been...

WWYDI you were playing hide-and-seek and one of your siblings/friends stayed "missing" for hours?

Ask them if they'd been to Narnia when they returned.

WWYDI you broke a window while playing cricket?

RUUUUUUUUUUUUUN!!!! WHERE IS THAT WARDROBE??

WWYDI suddenly acquired unusual archery skills?

Be sooooooooooooo happy!

WWYDI you were hag-and-werewolf-and-goblin-and-cyclops-hunting with the Pevensie boys and found Gollum?

O.o HE sure has traveled a long ways from Mirkwood...... Ummm, try to catch him?

WWYDI you woke up one morning and found all of the books in your house were about something to do with Narnia?

Not be surprised, and be very very very happy.

WWYDI Aunt Alberta asked you to babysit pre-VDT Eustace?

Do it, and go look at the painting in the upstairs bedroom.

WWYDI your bulldog objected very highly to something you said about its nose?

Laugh and apologize.

WWYDI a Narnian werewolf bit you?

Rrrrm, hope I didn't die and try to kill it?

WWYDI one of your sort-of-friends brought in two other friends, suspicious-looking and hooded?

Hope they turn out being like Aragorn and not the werewolf and hag!

WWYDI you were asked to judge between a false Aslan and the true Aslan that seemed very much the same?

O.o Concentrate. Concentrate hard, and ask Aslan to give me wisdom?

WWYDI you met a sly, catty..... cat, that acted like Ginger?

Assume that it's just bad-tempered and ignore it. Unless it was a Talking Cat, and then it's a completely different matter!

WWYDI you went to the zoo and saw a chimpanzee wearing a paper crown and slippers that curved up at the toe?