How Do I Get Rid Of Cat Urine Smell With Homemade Remedies?

We have a litter box in our bathroom, but the two cats that we have don't always make it in the box. To make things worse, the box is in our guest bathroom. To make things even worse than that, my teenage boys also use this bathroom and have not learned that the litter boxes need to be cleaned out daily. Ultimately, I need an answer to the urine smell. Ammonia is not a pleasant morning oder, not to mention the possible danger for my four year old.