The rules for claiming head of household and those of claiming a dependent are similar but different. Your claiming a child as a dependent does not mean you are head of household; nor does allowing you to claim your son as a dependent mean she loses the ability to claim head of household. I included the IRS Publication 501 which lists the rules for claiming the Head of Household status.
IRS Publication 501 - See page 8 for Head of Household Rules
https://www.irs.gov/pub/irs-pdf/p501.pdf
