I would say "This is for (kid's name), not you, so please don't be offended... and if you are, I'm sorry, but (kid's name) needs it more than your pride." Some people need to learn to swallow it.
Or maybe you could be like.. "Hey, my sister (or whoever) had this in her closet and we were trying to think of someone who could fit in it instead of throwing it out. I thought maybe you could?"