Protecting Our Children Comes First Act of 2003 - Amends the Federal criminal code to authorize officers and agents of the Secret Service, at the request of any State or local law enforcement agency or the National Center for Missing and Exploited Children, to provide forensic and investigative assistance in support of any investigation involving missing or exploited children.

Amends the Missing Children's Assistance Act to: (1) include among the purposes of grants under such Act coordinating the operation of a cyber tip-line to provide on-line users an effective means of reporting Internet-related child sexual exploitation in the areas of distribution of child pornography, on-line enticement of children for sexual acts, and child prostitution; (2) extend the authorization of appropriations through FY 2007; and (3) authorize increased appropriations for the annual grant to the Center for FY 2004 though 2007.