This is a really lovely idea, I love a good treehouse and all the objects work nicely with your composition
Like it has already been said, maybe adding another light source would help, maybe that hanging light on the right could be dimly lit so it would light up the rest of the room, but the candle would be the main focus. Maybe also look at your character again, her face isn't really giving off any emotion at the moment - i feel so sad for her that no one came to her birthday but we need to see that in her face too
maybe she could also we wearing a party hat?
Also, the top of that tree trunk looks a bit thick so maybe just break up those big branches a bit more so it's not just one big clump at the top - if you know what I mean?
This is going to look so beautiful in colour and i can't wait to see it! If adding colour scares you (it scares me when i come to adding colour) then just do bits at a time. Just start off with the tree trunk or something or the table with the cake on it, you can always sort out the lighting at the end when you're happy with the colours you've got
Good luck with it, i'd love to see your colouring process so we can help you along the way if you're having trouble!