Hi, I'd like to help you with your question.
It sounds like you are doing all the right things when it comes to helping your grandchildren. Having them see a doctor is a good step as you always want to rule out any physical problems. It is unlikely that both children would have a physical problem that causes bedwetting, especially at the same time, but it never hurts to be sure.
I am assuming that the mother is not involved for some reason with her children's issues. Is she unable to care for the kids, or is she unwilling? This makes a difference because the burden then becomes yours and the concern here is the level of stress this can cause for you. Are there any other adults involved that can offer you support? You also may want to ask the children's doctor for advice as well. They will be familiar with this issue and possible causes.
Since you have tried so many options, and assuming you are able to rule out a physical cause, there could be possible trauma the children suffered at some time. Any type of abuse, exposure to seeing traumatic events (such as violent parental arguments) or other trauma can cause children to develop bedwetting and other symptoms like nightmares. Even simple things like parents going through a divorce might trigger a reaction.
In this case, counseling may be necessary to rule out any psychological cause. A therapist can do a thorough evaluation and determine if there is possible trauma.
There are also some resources that may help you. One is called Waking Up Dry: A Guide to Help Children Overcome Bedwetting by Howard J. Bennett. Another is Seven Steps to Nighttime Dryness: A Practical Guide for Parents of Children with Bedwetting by Renee Mercer. Another product is a bedwetting alarm that you can use if all other options are ruled out. It's called Malem Ultimate Bedwetting Alarm - Blue 1 Tone w/Vibration by Malem. You can find these resources on Amazon.com or your local library may have the books available.
I hope this has helped you,