I have a small area in my backyard where I have put in native grasses
(indian grass, switchgrass, little bluestem & big bluestem) It is
coming up slowly, I planted from seed about 8 weeks ago. Now the
tallest seedlings are about 3/4 to 1 inch tall.

Problem is there are a number of weeds in the area. Do I pull them,
or just leave them and assume the grasses will eventually grow over
them and make them go away?

It is a 20 foot by 10 foot area that I had always just let my regular
grass grow long, the kids love to catch insects in the area, so I
finally bought to some native grass seed, it should be cool once it
gets tall..

I have a small area in my backyard where I have put in native grasses
(indian grass, switchgrass, little bluestem & big bluestem) It is
coming up slowly, I planted from seed about 8 weeks ago. Now the
tallest seedlings are about 3/4 to 1 inch tall.

Problem is there are a number of weeds in the area. Do I pull them,
or just leave them and assume the grasses will eventually grow over
them and make them go away?

Hand weeding's probably the easiest, especially in 200 sq ft.
Here's some help with your project:
http://dnr.state.il.us/conservation/...irie/table.htm
http://www.inhf.org/prairiemgmt.htm