Removing urine from a mattress is not that uncommon. Most of us with children have had to deal with problem sooner or later. Your child crawls on your bed and pees, or the babies diaper springs a leak while bouncing around on the bed. I deal with it on a regular basis as an upholstery cleaner in Tampa FL. The good news is that today's mattresses are more stain resistant then the older silk top models of 15 years ago. Most mattress manufacturers have moved away from natural casings to synthetic upholstery. This means that it's easier to get clean.
The problem is if the urine goes unnoticed for too long it will work it's way into the padding under the surface and it will be a lot more difficult to remove. If you can get to it right away you wont have much of a problem with it.
Removing urine from a mattress:
1) Blot up the excess urine.
2) Rinse the mattress with room temperature water mixed a few drops of Hydrogen peroxide 3% solution.
3) Wait 30 seconds.
4) Dry with another clean white towel.
5) Sprinkle the mattress with some baking soda and let it sit for a few hours, until it has dried out.
6) Vacuum the baking soda and your done.
If the stain remains it's time to call a professional upholstery cleaner who knows how to remove urine from a mattress.