Awww, bless! I think it’s kind of metaphorical because not every tree looks the same!
I like your idea of drawing around the hand or could they draw a tree top on green paper for you to cut around afterwards? Or they could do a collage tree; again separate for you to cut out and stick on the trunk. You could even make the collage from real leaves.
Your post really made me smile and I know that whatever you do will be lovely.
How about a foot, just to be completely different…it might look like a tree blowing to one side in the breeze and perhaps your LO wouldn’t mind their foot in the paint trough.