I would harden them off gradually and move outdoors - those temps should be plenty warm enough for them, however you will want to bring them in if temps should drop below freezing at night.

A 10 cm container is pretty small and I'd advise potting up although now is NOT an ideal time to do so. However, if you are very careful and do not overly disturb the rootball, it can be done. For those size trees, I'd start with a nursery gallon, or a container with dimensions roughly 6 3/8" wide by 7 1/4" deep, holding approximately 3 quarts. This should accommodate for them at least one full growing season or about 12 months before they fill the pot and start becoming rootbound. Use a very textural, durable bark-based mix, not a very finely textured potting soil like Miracle Gro.

Can't tell you how often to water - there is no single correct answer to that question :-) The soil should not be allowed to dry completely but stay just evenly moist, not wet.

FWIW, this is a very slow forum and you will get more, faster and much better responses if you post any additional questions on the Maples forum. Lots of knowledgeable folks visit there daily.