Raising Baby Zebra Finches?

I have a male and female zebra finch. In the cage there are 2 open half shell-shaped nests and one enclosed peep-hole type nest. Usually every other day I remove the eggs, but recently I couldn't get to the ones in the enclosed nest. Then I noticed they were each in that nest more often, together at night, and usually at least one was always there during the day.

I stuck my finger in the opening to check for eggs, it had been a couple of weeks and felt a live baby bird, to my surprise. I called the pet store and they basically said to leave it alone, the parents will care for it. I can see it moving in the nest and I can hear it's attempt at "trumpeting". I'm guessing it is a male. I can see grey fluff, but it hasn't come out yet.

What are the stages of egg laying to hatching and the stages of development and growth for these birds? (How old do you think it is and how does it grow week by week?) I've read both to separate and not to separate them "at the right time", please explain. What can you tell me that I would not think to ask? I've had 2 pairs (in separate cages) for 8 months now, but I am still new at this. I really do not want any more babies.

By Kim