Dave Greenman

i'd guess they're birch
either
Betula Papyrifera, looking at the lenticels

BUT if the lenticels 'stretch' or elongate sideways
i'd say Red water birch Betula occidentalis

Hi,

try increase temp a little if you can, tho not entirely necessary

when seedlings are about 2cm tall prick them out into 9cm pots and return to window sill for a few weeks
as they get bigger and the weather gets warmer (Not too familiar with Sweden )
they can be hardened off over the space of about 2 weeks (they are quite slow to lignify and can flop over ) , and keep increasing pot size when needed !!!
they can be left out until the first frost but give them some protection at least over their first winter, plant out in second spring when around 2 foot tall

good luck i really love these trees

Propagation
Seed - sow early spring in a cold frame in light shade. Seed can also be sown as soon as it is ripe in a greenhouse. Germination rates are usually very low, two months cold stratification might help. Prick out the seedlings into individual pots once they are large enough to handle and grow them on in a cold frame for their first winter, planting them out into their permanent positions in late spring or early summer. Plants will require some protection from cold and spring frosts for their first year or two outdoors. If there are sufficient seeds, they can be sown in a lightly shaded outdoor bed in late March. Grow them on for two years in the seed bed before planting them out into their permanent positions in late autumn or early spring.