She's ba-a-a-a-ack! SiT is here without any brilliance of her own, just with her scalpel and magnifying glass....
That big, beautiful tree has a sign on it that I can't make out; not even what language it's in. Maybe, Danger, No Climbing? Is that the uncontrolled and threatening form of nature that the LWB refers to, which it is natural for children to seek out and grasp? It would be more natural for children to climb trees, than to dress up for "command performances" by adults. I quite like your role-playing, dressed-up-for-someone-else idea. It fits in with, at least, my take on the Page of Pentacles: forming and moulding of kids contrary to the patterns of nature.
The boy does look ghostly--well, both kids do, being in that misty vapour, and in a different light-source than the one that the tree has. What is the boy doing in his pyjamas, outdoors? Sick-bed attire, and he has since died? Yet was expected to play the violin, in spite of being poorly? Or is it an obsession, reminiscent of The Rocking Horse Winner? The fiddle is considered, in some places, to be a musical instrument that the devil plays. (Have you heard the fiddle tune "The Devil Went Down To Georgia?" It narrates a fiddle-playing contest between someone and the devil. You can almost smell the strings smoking, so fast and intricate is the playing!) There would be the boy's fire element.
The item that the little girl is holding doesn't have stops like a (modern) flute does, and the bottom appears solid, not hollow. It is also creamy white like white wax or paraffin, and not metallic or even wooden. I really, really think that that is a long taper candle that the little girl is holding, twined with a decorative vine, that she will light at mass. It has a long wick on the end. This is her wand: fire waiting to burn. I guess she's going to her first communion, like Bumblebee says. Now, I'm not Catholic, so someone Catholic please correct me, but the little girls are dressed like brides at first communion, sort of to mark their "engagement" to Christ/Christianity; after which if they become nuns, they become "brides of Christ". Pure, virgin, solo. Brotherless. Even so, you suggest that her brother is still "present" within her, and she dances to his tune, as it were. The LWB does speak of hidden passions, especially in young people.
(I did read The Turn Of The Screw long ago, but I guess I wasn't good at nuance, because even when I went back through the book, I never found what it was that had set the governess off in a flap. And she wouldn't say what the problem was in so many words, either. All I remember is not understanding what was going on. Feeling like a kid with adults talking over my head! Unless Jane Eyre counts, that's the only gothic novel I've read.)