Costco mattresses are good but for a twin mattress for a toddler I'd go with Mattress Discounters. We bought a nice twin for DD three years ago and it is as good as new and super comfy. Just make sure to invest in a mattress cover.

Costco?

Mattress Land on Rt 7 in Falls Church. It's across from Toys R Us. My designer told me about this place. Andre is the owner, and he's awesome. Great prices right off the bat. Check out the yelp reviews.