、、。
Chicks that have just hatched have wet down feathers, but after about two hours they dry out and become fluffy.
Literal
Hatching-after not-long chicks [topic-は], down-feathers [subject-が] are-wet [but-が], 2 hours [even-も] do-[when-と] dry-and fluffy become.
Several small structural things worth noticing. まもない ('shortly after,' 'not long since') attaches to 孵化後 ('after hatching') to modify ヒナ — the whole phrase 孵化後まもないヒナ is a relative clause meaning 'chicks that are not long after hatching.' ２時間もする uses the focus particle も with a time expression to mean 'as much as 2 hours' / 'about 2 hours,' often with a nuance of 'surprisingly little time' in context; the following ~と is the 'when/automatic consequence' conditional. フワフワ is a 擬態語 (state mimetic) for a fluffy, airy, soft texture — common with things like fresh bread, cotton, clouds, and down feathers. The final ~になる marks the change of state into 'fluffy.'