You planted it right as evidenced by the sprouts. It's when you planted them is the problem. Japanese maples are outdoor plants and need some dormancy. You're going to have to keep them alive all winter indoors and then throughout the normal growing season next year. I've never tried it myself. Not an easy thing to do and I've raised a lot of Japanese maples from seed. Usually the seeds are planted outside in early spring.

Mike

Maple seeds still fresh and soft germinate best, so it would seem late summer/autumn would be the preferred planting time. With the newly planted seeds getting a cold period afterward. Maple seeds that have instead become hard and brown may take a long time to come up.

For the seedlings shown here to be maples and coming up like this they would have had to have been given a dormancy period beforehand - I wonder if they really are maples (Acer sp.).