A 'Mikawa Yatsubusa' I'm still not sure what to do with. Might plant it in the ground to use as a mother plant, or try ground-layering it above the graft. LOVELY colors though!
I also have a 'Sharp's Pygmy,' but it has not leafed out yet. TBH I am getting a bit worried about it- seems like it suffered a lot of die-back over Winter. Only the lowest buds are pushing at all, and most of the upper branches look dry & brittle.
You probably already know this but, because that seedling is the product of sexual reproduction . . . even if you know the cultivar of the parental tree, what you have there is an unnamed cultivar of Acer palmatum.
If you do already realize this, maybe someone without the knowledge will benefit from it being stated.