What percentage of marriages involve existing children, either from
previous relationships or from within that one?

Hi vtsstu,

Here you go, click on the links for alot more data.

The Salt Lake Tribune
Sunday June 29, 2003
"Today, the number of U.S. children living in families with two
married parents stands at 69 percent, far off the 85 percent of
children reared in such families in 1970..."
http://sfhelp.org/01/research4-mrrg.htm

United States
S0901. Children Characteristics
Data Set: 2004 American Community Survey
Children under 18 years in married-couple family household: 50,060,337
http://factfinder.census.gov/servlet/STTable?_bm=y&-geo_id=01000US&-qr_name=ACS_2004_EST_G00_S0901&-ds_name=ACS_2004_EST_G00_&-redoLog=false

United States
General Demographic Characteristics: 2004  
Data Set: 2004 American Community Survey 
Family households (families): 73,885,953
Married-couple families: 55,223,574
http://factfinder.census.gov/servlet/ADPTable?_bm=y&-geo_id=01000US&-qr_name=ACS_2004_EST_G00_DP1&-ds_name=ACS_2004_EST_G00_&-_lang=en&-_sse=on

United States
S1201. Marital Status  
Data Set: 2004 American Community Survey
http://factfinder.census.gov/servlet/STTable?_bm=y&-geo_id=01000US&-qr_name=ACS_2004_EST_G00_S1201&-ds_name=ACS_2004_EST_G00_

United States and States
R1102. Percent of Households That are Married-Couple Families With Own
Children Under 18 Years: 2004
Universe: Households
Data Set: 2004 American Community Survey
http://factfinder.census.gov/servlet/GRTTable?_bm=y&-geo_id=01000US&-_box_head_nbr=R1102&-ds_name=ACS_2004_EST_G00_&-format=US-30

I was glad to find this for you. If you have any questions, please
post a clarification request and wait for me to respond before
closing/rating my answer.

Thank you,
hummer

I searched the census bureau and also used the Google search terms:
percentage marriage with children