According to the National Incidence Study of Child Abuse and Neglect, published in the United States in 2010:
Children living with their married biological parents universally had the lowest rate [of maltreatment], whereas those living with a single parent who had a cohabiting partner in the household had the highest rate in all maltreatment categories. Compared to children living with married biological parents, those whose single parent had a live-in partner had more than 8 times the rate of maltreatment overall, over 10 times the rate of abuse, and nearly 8 times the rate of neglect.
The traditional nuclear family is still the best choice, and introducing partners into a childs home is putting them at extreme risk.
https://www.acf.hhs.gov/sites/default/files/opre/nis4_report_exec_summ_pdf_jan2010.pdf