How to Clean Mattress That Got Urine on It?

We use the pet enzyme stuff (Nature's Miracle, or there are other brands) that you can buy in big (gallon) containers at Petco or other pet stores. It really is a miracle cleanser. I like the original nature's miracle the best, although it's a little pricier than the store brand. You use the enzyme cleaner to completely soak the area - don't be shy with it - and let it dry completely, which for a mattress can take a couple of days. (My son recently had a similar accident. There are a lot of bad mattress covers on the market!)

I have four children who have at one time had bedwetting issues due to bladder problems, so it is a constant challenge for us! I have found that as much as I hate lugging them outside that the best way to make reasonable improvement is to use a hose outdoors and a whole gallon of bleach (Clorox is more friendly to the environment and the kid's health) per mattress and rinse for a very long time then air dry in the sun for atleast 24 hours to help dry and sanitize. The sun is a great sanitizer!!! Afterwards, you can spray them with fabric softener (Unscented, if your kids have allergies) to help keep them from absorbing more odor.

Target carries vinyl waterproof ones. Did you have that kind or the cloth kind? If you didn't use the vinyl you may want to try it in the future. Also, Melaleuca carries a sanitizer that you can spray through the mattress and it totally kills the bacteria and smell plus its totally chemical, toxin free. I would not use bleach or clorox because they sleep on the mattress and bleach is highly toxic even after it dries and is absorbed. Very very poisonous. Do not use it. If you want I can get you the melaleuca stuff or else you could look into other options not so toxic that I don't know of. Good luck.

Try a paste of baking soda and water.

I don't know how big her mattress is but my great grandmother, grandmother and mother have always said that the sun is the best way to remove stains and smells. I would maybe just put it out on a porch for some sun (I don't know that I'd hose it down).
Just one last thing, if you buy the vinyl mattress cover, I suggest a soft and thin throw (they are usually smaller than a mattress) for in between the cover and her bed sheet. It will make it more plush for her to lay on and will be easy to throw in the wash. Wish you the best....sounds like you have lots on your plate.

Not sure how to get the smell out, maybe use a pet cleaner.
As far as the mattress cover goes, I bought some at Kohl's that go around the entire mattress and zip up. Try those.