My experience is to add a collar as soon as the kids are no longer at risk of getting stuck in fences on the daily (ok, good practice says our goats should never get their heads stuck, but I see it happen to even the most perfect goat owners.)
My opiniom is that it is never too early to put on a collar and practice leading around, just remove collar when kid is turned back out. I would start by leading mama and kids together, as that creates a hopefully stress free experience.
I haven't done training to the stand yet, but I would like to say its good to begin that early on. You want the kid to trust humans to put it on stand and touch various parts, including manipulation of hooves. My particular stand is large and a small breed kid would simply slip out, this may be a factor for you.