What breed is this?

What does the comb look like? Possibly Sicilian buttercup

 

I have the same question ... I am also getting worried I might have a rooster, thoughts?

 

I agree with ChickenLover that it could be a Sicilian Buttercup. It could also be a Golden Campine.

 

How old are they? It's hard to tell their sex now, but the brownish-colored chick gets me worried too.

 

They are 3 weeks old

 

I think the brownish-colored chick is a Barnevelder or a Dark Cornish, maybe even a Welsummer, although I'm not entirely sure. The other chick is much more difficult to tell, as many chick have a similar sort of pattern (possibly a splash Blue Andalusian?). Pictures of the chicks when you first got them (from MPC) are also very helpful, as you can more easily match them up to the breed. They could be any sex though, you'll just have to wait until they're older for that information.