In the very early days say up to about 10, unless you really know your ferrets well it is probably better to err on the side of caution and leave the kits alone. In well over 60 years of breeding ferrets I have never lost a litter due to the jill killing them.
All the jills are well handled and above all when they first have the litter it is essential they have plenty of food and perhaps for the first couple of days a small amount of milk. When I first check the kits I make sure the jill is drinking some milk then normally use a small twig to open the nest to see whats there, only a quick look. After a couple of weeks I look at the kits and have no trouble with the jills. You will as I said earlier get the odd one that can get really quite nasty when she has kits, but this is rare and she will settle down as the kits grow. I think the killing of kits by jills is a bit exagerated probably comes from the days i just about remeber when ferrets were treated quite badly, fed nothing but bread and milk, and then only sometimes. Hungry nervours jills can then become over protective that plus hunger equals disappearing kits!