M.S.
While reading your post all I could think of was that it sounds like your mom might her own personal stuggles.
By the fact that your kids are crazy about her means that when she is able to be with them she connects with them, which is awesome.
I understand your concerns very well, it could me my story.
I used to expect way to much from my mom......I mean come on you are my mom you are suppose to rule the world ,take care of me, be strong, never faulter, be perfect, have all the right answers, be there for me always. The reality was
she never was ever. I expected way to much from her.
She was human but I didn't allow her to be. I only expected perfection from her. Your kids are smart they get it already. They love grandma just the way she is even if it is only a few times a year......the hardest part is perhaps for you to except her just as she is.
Don't explain anything to your kids. They love her already, if you start judging her to them, they will start seeing her differently. You have to power to keep her a positive person to them. I know I find it hard to seperate
my hurt feeling because of my mom and not manifest it on to my son. Just feel blessed that she is there sometimes and that your kids love her. That is way better then nothing at all.
warm wishes,
M.