Should we let our children visit their dying grandmother?

Yes. I imagine their grandmother would be very happy to see them & they should have the opportunity to see their grandmother again & say goodbye to each other. Many in our society fear death & dying & live in denial, but dying is a natural process and i believe it is healthy & educational for your children to spend time with their grandmother now. They'll likely treasure this opportunity when older.
It depends. It depends on the age of your child, how the grandparent looks, and whether or not they had a close relationship before the grandparent was sick. If the child is too young to understand, the grandparent looks to "scary" because of the illness of treatments in the hospital, or they never knew the grandparent then i would say no. Use your best judgement - if it seems like a bad idea, don't do it.
Yes. Yes, but don't force them to go. If the children and grandma have an existing relationship, it will be good for both of them to have a visit. Tell them grandma is sick, describe any equipment that might be in the room, but don't dwell on it. Keep the visit brief, and honestly answer any questions they may have.