I imagine that while they're young, they have to be taught how to kill, fight properly, as well as any other sort of survival tool until they get to a certain age when they are cast out for a certain length of time and are required to survive. When they come back, they are much tougher, and probably also have to think about some sort of philosophy they hand down.
In my story, Mara was never raised as a Crimson Maiden, so when she arrived in their lands, she underwent a psuedo test, where she was out in the wilderness, not just to survive, but to fight, and in this case rescue someone important to her. I imagine Crimson Maidens would accept another as one of their own as long as they show some sort of trait related to their kind, and then they are allowed to under go a partial/altered proving.
I see a proving as "prove you are indeed one of us". It probably also gives honor to the family, and the more Crimson Maidens a family has and that get through alive, gives them more honor.
Of course a place like the Jungle of Perils/Crimson Wood is really dangerous, so having to survive out there as a kid would likely result in some fatalities.
On the other hand:
Don't forget the wiki also states that the only time anyone in the lower regions of Felarya ever run into Crimson Maidens are the children who are out on their proving quest. However, it doesn't state that they are indeed children - implying Crimson Maidens may also go through provings when they are full adults too. I think that means there is something that has to do with skill in certain areas, and that they need to meet minimum requirements to go on this proving...
Also that Crimson Maidens go south, or are seen in the south implies they have to travel, and are not restricted to where they have to go. Here's the question though: What exactly are they proving, on a proving quest? That they are an adult/mature, that they are skilled fighters, that they have found some truth about the world?
We know they are fierce warriors mainly, but if they also have scholarly and some philosophical aspects to their society, knowledge and secrets could be a big part of their proving. Crimson Maidens do feel like some ancient society...