Just heard a close neighbor has amyotrophic lateral sclerosis. how do we explain this condition to the kids?

Still same person: As far as the disease itself is concerned, let them know that it is weakening their muscles, which affects everyone differently, causing difficulty walking, speaking, breathing, or using their hands. Also let them know that it is not currently curable, and as such will get worse as time goes on. Most importantly, emphasize that no matter how they look, they are still the same person inside.

Weakness: I would explain that it means he will get weak over time, may eventually need a wheelchair, and that his doctors will do everything they can to help him. I would tell them they can't catch it from him, and it is a rare condition, so it is unlikely they or their family members would develop it.

Simple approach: Some folks acquire diseases that can result in weakness and difficulty getting around. They will need help on occasion, and kindness. The disease is not infectious, and the kids will NOT catch it, and are very unlikely to ever get it. OK to have contact with the neighbor. There will be no bad behavior or emotional problems in most cases, but depression is to be anticipated.

Depends on the kids: Before age 8 or so kids are not capable of processing complex explanations.A simple short, s/he's sick with an illness that causes weakness, would do.Expect new questions later but always keep it short & simple. Kids 9-15 can handle more.They can sit though a brief explanation of the illness. By 16+ they can be treated like an adult.