I think your children will be fine. They'll probably ask questions, so you can let them know ahead of time what to expect in very simple terms. They'll see some tubes and machines that look funny but they're intended to help Uncle Larry breath and get medicine into his body because he's very sick right now. Kids adjust very quickly, so if you're worried that they'll be traumatized I have a feeling that they'll be fine.
However, I think you need to call the hospital first to see if the children are even allowed to visit. Many hospitals are not allowing children in as visitors into ICU's, maternity wards, hospices, and other wards because it's the middle of the seasonal flu season and because H1N1 flu is still a huge concern. It may be a moot point on whether or not you bring the children.