A.T.
I'm an interior designer (when I have a job);) and I sold furniture and linens as I worked my way through college - so here's my 2cents.
Don't get a little kid a pillow top - the kiddie sheets with their favorite characters won't fit - also the pillowtop tends to get saggy and lumpy and they are usually too tall for a little kid to get in and out of bed easily and they usually look goofy on a kids bed - cover up too much of the headboard.
Buy a known brand mattress and box spring with a warranty and medium firmness - no pillowtop. Mattresses last longer when used with the manufacturer's box spring - don't mix -n-match.
Then invest in a waterproof/allergen proof cover that zips over the entire mattress and a good quality 100% cotton mattress pad (maybe 2 if kiddo still has accidents). You can often get really good quality Twin size stuff cheap at Marshall's or Ross because it's a less popular size.
I would leave the mattresses unwrapped and uncovered for at least a few days with the window open or a fan blowing to air them out before anyone sleeps on them - bedding sold in CA is treated with flame retardant that can be a bit stinky and fumes can make you sneeze or itch. Wash all sheets and bedding before use to remove sizing and other chemicals and soften the fabrics and make them smell familiar to your child.
Finally - go ahead and cut off the Do Not Remove tags and enjoy!