Hi, I had the same problem, my cat all of a sudden starting peeing on top of my daughters bed, it was around the time we had a new cat (well it was a stray kitten that the neighbour put over our wall) and I guess this intrusion made Lucky jealous, we had other cats too but they were here before Lucky. I was going crazy, its one thing peeing on the carpet or floor, but it seemed he would save it up and pee buckets on the bed and it would soak through to the mattress as well. My vet suggested putting his food on the bed a few times as normally cats won't pee where they eat. I did this for a few days and it really did work.
Sometimes it is behavioral as suggested above, but sometimes it is a signal that something is medically wrong. Something as simple as a urinary tract infection or as serious as kidney issues. If nothing in her life has changed to cause a behavioral problem and if you are keeping her litterbox clean (have you moved the box? moved something near the box? changed brands of litter?) If all of these things can be ruled out, I would take her to the vet for tests. Incidentally, even though I don't have small kids, I don't appreciate pee on my couch either. LOL