I replied yesterday morning but my post still doesn't show up... So I do it again.
All airlines offer assistance to their passengers. Most of the time it's indeed a wheelchair, sometimes they have those little golf carts, but it depends on airports.
What I would suggest is they get in touch with KLM, by phone first, so the agent can put a note in their file. And when they show up at the airport, upon dropping their bags off, they mention it again to the agent. They will be accompanied up to the plane. Passengers with assistance board among the first ones. I'm not sure if you can ask for assistance only in a specific airport (AMS in your case), but you don't lose anything by asking. There is no sign when they step of the plane, but someone waiting with wheelchairs, right at the door of the plane. If there's nobody, they should mention it to the flight attendant so someone will be called.
I cannot post any link yet, so google "special assistance by KLM care" and "KLM medical assistance": you should find all the information you need.