So, I went ahead and put my older chicken in with the babies today and watched. First, it pretty much ignored the babies and focused on trying to escape the brooder. Unfortunately when it finally started paying attention to the babies it started pecking at their feet, so I intervened three times and then took it out of there...Any thoughts?
Chicks will peck at everything, a few times.
If it breaks the skin to draw blood, or it tugs on the toes and knocks the babies over, then of course you have to intervene.
Otherwise, I might just let it keep trying to peck toes for a bit--it will eventually figure out that they are not good to eat.
There's a point where it's best to just let them work it out for themselves, and there's a different point where you have to intervene to prevent injury--unfortunately, it can be hard to recognize what point is where. (And harder yet to try to guess it from a distance!)
So I don't know whether to advise lots of short sessions, or a long stretch of watching it try over and over to eat toes until it gets bored.
Chickens do peck things to learn about them, and they do peck as part of their communication, and they do peck to eat--so it can be hard to guess what is going on at any specific instant.