FAFSA doesn’t require that a child be actually living in the same residence, only that the parent(s) provide more than half of the child’s support during the reporting period.
From the FAFSA instructions:
“To determine whether to include children in the household size, the “support” test is used (rather than a residency requirement) because there may be situations in which a parent supports a child who does not live with the parent, especially in cases where the parent is divorced or separated. In such cases, the parent who provides more than half of the child’s support may claim the child in his or her household size.”