Japanese Maples: Raintree has a sale on 2' or 3' Red JP maples, 1 by doss

doss wrote:
The only way that I've killed them is to put them into the sun too fast when they have been under shade cloth. They call it 'hardening' when they gradually move them into a new position. Wind can be a problem in addition to sun. If you have them in pots any of them need to be grown in part shade in order to keep from burning too much.

The thing about seedlings though is that they are the tougher plants. Yours should be happy in full sun in the ground if you move them there gradually.

It will be a great experiment anyway. I had to do the same and came out with one incredibly beautiful seedling, luckily. I've posted this photo before but I wanted you to see that seedlings can be really beautiful - of course they are all different and I've seen at least one 30 feet tall.

You won't have risked too much anyway. A good start.