Answer: The McKinney-Vento Act explicitly authorizes the use of subgrant funds for:
“The provision of tutoring, supplemental instruction, and enriched educational services that are linked to the achievement of the same challenging State academic standards as the State establishes for other children and youths.” 42 USC 11433(d)(1).
Therefore, you could probably make the case for the use of McKinney-Vento funds for this activity, as long as it is linked to the achievement of the “same challenging State academic standards as the State establishes for other children and youths.”
In addition, the Act authorizes the use of funds for: “The provision for homeless children and youths of before- and after-school, mentoring, and summer programs in which a teacher or other qualified individual provides tutoring, homework assistance, and supervision of educational activities.” 42 USC 11433(d)(8).
It is unclear what activities or supervision is included in the field trip, but that might be another provision to support it.