That doesn't really seem a putdown to me at all. In fact, my son is 6 and LOVES his kilt. He even asked me Tuesday night if we could "kilt up" for the hockey game. Of course I said yes. But even though he loves his kilt, loves to see me in my kilt and knows it is a kilt (and generally refers to it as a kilt)....he still calls it our "man skirt" sometimes, because he likes to remind me it is a lot like "mommy's skirts".

I think you handled the situation perfectly. When young children ask, they are just interested. It's best to handle it the way you did and let them go away better informed. Well done.