UN Convention on the Rights of the Child: Article 11: Illicit Transfer and non-Return of Children , Article 21: Adoption, Art 35: Prevention of Abduction, Sale and Trafficking
Abduction
Adoption
Trafficking
Under the Rome Statute, enslavement is a specified Crime against humanity. A subsequent section explainst that: '"Enslavement" means the exercise of any or all of the powers attaching to the right of ownership over a person and includes the exercise of such power in the course of trafficking in persons, in particular women and children;'
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Subject searches to use in SOLO
Unaccompanied immigrant children
Children's rights
Emigration and immigration law
Research Handbook on Child Migration by 
International Migration of Children for a Better Life by 
The Child in International Refugee Law by 
Child Migration and Human Rights in a Global Age by 
Unaccompanied Migrant Children : social, legal, and ethical perspectives by Subject searches to use in SOLO
Intercountry adoption - law & legislation
Conflict of laws - adoption
Adoption - law & legislation
Surrogate Mothers - legal status
Surrogate Motherhood - law & legislation
International Adoption: Global Inequalities and the Circulation of Children by 
The Hague Child Abduction Convention by 
International Surrogacy Arrangements : legal regulation at the international level by Suggested subject searches to try in SOLO:
child trafficking victims;
Enslaved children
Child labor
children - crimes against;
Do not neglect the broadest children (international law) as trafficking may feature as a chapter within general overviews.
Below are some examples of books on the topic:
Trafficking of Women and Children : Article 7 of the Rome Statute by 
Child Slavery Now a contemporary reade by 
Medical Perspectives on Human Trafficking in Adolescents by 
Trafficking of Children for Sexual Exploitation : public international law 1864-1950 by