While the game itself hints that small downed UFOs make a good mission for bringing rookies along, this gets farther away from the truth as the game progresses. Later in the game even the smallest UFOs contain some pretty dangerous alien types (and in surprising quantities, at that).
I personally find that the best missions to bring rookies on are council missions (which are pretty rare, but always have a tendency to contain weaker alien types, usually Thin Men) and terror sites. While Chrysallids are initially intimidating, the farther along the game you are the more weapons you have that can make short work of them. They also have a tendency to come running at you, which is very helpful in aiming properly.
As a final point, consider the fact that with a good enough squad, you can probably keep a rookie alive and let him have a kill in almost any sort of mission. Just try not to take them on missions that you know are challenging. Also, explosives are your friend when you try to net a rookie an easy kill.