newbie questions

Hi, I just started a new lawn about a week ago.
I can see young grass now. They are about at most 1 inch above the ground.
However, I found some bare spots and want to cover with more seeds, but I am afraid to step on the new grass.
Any suggestions when I can do that?

Also the grass is not dense as I thought it should be even I double the recommend amount of seed at the very beginning. (I covered the seed with a thin layer of loam)
I am thinking about overseeding, which requires using roller to get the seeds into the soil? How can do that without damage the young grass?

The last question is I bought some loam to cover the seed at the beginning.  Yesterday we had a storm, I noticed that after the rain there are some large spots appears to be white-ish (kind of washed-out).  How could that be?

Thanks for your help.

bare spots:  wait.  they are probably late bloomers! (or sprouters in this case)

not dense:  wait.  it will fill in with time.

overseeding:  wait.  you probably won't need to do it at all!

washed out:  not sure what this is all about, but I suspect that if you wait, it will all probably turn out just fine.

Thanks, Paul.
I guess I am a little nervous since it will turn cold soon (in Boston). 
When do you think I can walk on the grass, 3 inches, the mowing height?

Wait for it to get to 4 inches.