How about Pub 501 - specifically the qualifying child of more than one person

Sometimes, a child meets the relationship, age, residency, support, and joint return tests to be a qualifying child of more than one person. Although the child is a qualifying child of each of these persons, only one person can actually treat the child as a qualifying child to take all of the following tax benefits (provided the person is eligible for each benefit).

The child tax credit or credit for other dependents.
Head of household filing status.
The credit for child and dependent care expenses.
The exclusion from income for dependent care benefits.
The earned income credit.

https://www.irs.gov/publications/p501#en_US_2018_publink1000220917