This can be a serious & delicate problem for children, I speak from experience, I had a problem with pissing the bed untill I was like 11 or 12. There was absolutely nothing I could do about it, I'd wake up & it'd be too late. The bigger of a deal you make out of it, the worse it will be. I'd say the only preventative measures you can take is limit the fluid intake a couple hrs before bed, especially things like milk & put a liner on the bed. I can remember being sooo embarrassed and getting yelled at. I wouldn't stay the night with friends because I was afraid it would happen. It didn't happen all the time but, it did happen. I'm very fortunate, both my children 4(today) & 5 have been very very good at going to the bathroom & have been that way since around 2yrs old.