The 1st child will be a girl 1/2 of the time.
The 2nd child will be a girl 1/2 of those times.
The 3rd child will be a girl 1/2 of those times.
The 4th child will be a girl 1/2 of those times.
The 5th child will be a girl 1/2 of those times.
The 6th child will be a girl 1/2 of those times.
The 7th child will be a girl 1/2 of those times.
The 8th child will be a girl 1/2 of those times.

Probability: (1/2)^8 = 1/256.

[But you must take this, as well as many problems
found in mathematics courses, with a grain of salt 
in reality.  In this case, it is true that some men 
produce more female-sperm (sperm that would result 
in female offspring) than male.  Also, some women's 
eggs are predisposed to accepting only female sperm.  
Male-sperm gets rejected.  So to compute the actual 
probability accurately for reality, we would have to
know the probability of men and women being female-
producers].

Edwin