As far as age of chicks, anywhere between 1 and 3 days old is fine. Some hens will even take them older.
As far as her being broody only 2 weeks? It will depend entirely on her. I have my best luck when the hen has been broody at least 16 days, but I've also had one hen who accepted a chick after being broody only 5 days.
For what it's worth, my experience:
Hens broody over 16 days -- 4 out of 5 times success. The one rejecter was a real pill of a bird, and I eventually culled her for behavioral issues.
Hen broody less than 2 weeks: 1 out of 3 success.
I agree with the others, put them in at night. Also, when you insert them, don't show her the chicks, try to slip them in behind her, so she feels them under her first.
She will most likely start pecking when you put your hands near the nest. I use one hand to distract her -- I wear a glove and let her peck it -- and then use the free hand to quietly tuck the chicks in behind her. You can remove any dummy eggs at the same time. I usually put a broken egg shell in there with the chicks just to complete the illusion, but it's probably not necessary.
Good luck!
