For an enzyme cleaner, Nature's Miracle is the best (I have four kids and five dogs, so I know whereof I speak!). You can usually find it in the pet section of any discount store. Follow the directions on the bottle; I was using it wrong for a long time and, surprise surprise, it works better if you use it right! A great thing to do, if you can, is lay the mattress in the sun for a few hours.
Then, like somebody said, run to the store and get a waterproof cover for the mattress (don't put it on till the mattress is 100% dry or you add a mildew problem to the urine problem!) and one of those wraparound mattress things that's like a diaper for the mattress. They're super absorbent. It's what we use for my 10 yo SS who is a nearly-nightly bedwetter and it holds an amazing amount of liquid. Because honestly, after it's been wet on several times (esp. when the day comes that you don't notice it right away and it sits for a few hours or all day), a mattress will never smell right again, no matter how much stuff you throw at it.