Japanese Maples rarely produce seeds exactly like the parent plant. Often its not even similiar. For example, from a Bloodgood I might expect more than half the seedlings to be green, such as in a generic green japanese maple. You just might get a couple that are similiar to the bloodgood out of several hundred and maybe a few more that are red.

The seedling you showed us looks more like a full moon maple(Acer Shirasawanum),instead of an Acer Palmatum which your parent plant is. I am not too familiar with anything besides AP's so someone else might give better insight. I don't know if the two cross polinate.

All maples have opposite branching. If it has alternate branching it is not a maple.

Wrong family....you've got a Kalopanax septemlobus

Ahh, I guess the soft spines gave it away as well. I forgot about the oppsoing branching.

Kalopanax septemlobus. Wow, good ID. From what I've now Googled, that looks like a very likely match. Looks like the fruit capsules have two seeds, and I have two plants very one another. I have no idea what it's doing in my little yard--I don't think any neighbors are growing it nearby. Not your average Chicago shade tree. Seed must have hitched a ride in somehow.

This is an unfamiliar tree to me. Looks quite interesting though. And maybe it doesn't need to be growing seven feet from the the Forest Pansy redbud. I'll have to think about where I want it.

It's one of many plants who's drupes are much enjoyed by birds.