As you have stated that is is age 6 it is unlikely she would win an item not as described dispute.
If she hasn't yet asked for a refund but is just complaining you can either offer a refund on return (in condition it was sent) (you do not have to refund her return postage as the error is on her part for not reading the listing) or you can do nothing and see if she calms down.
If she opens a dispute you can either offer the refund as above or let it go through dispute. As the listing clearly states age 6 paypal will more than likely find in your favour.
Of course she can leave a neg FB - so it's up to you how you proceed. I'd keep all correspondence short, polite and simply stating facts.