Preferential treatment on its face is not permissible.
In your situation, by providing special consideration to parents, your employer is probably seeking to fulfill its duty to accommodate, as parents likely do not automatically receive the day shifts. What is not apparent is whether the person being provided the day shift as “first priority” had to first request the accommodation from the employer, explaining why he or she needed it and provided supporting information.
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Sounds like discrimination toward people who do not have children or parents with older children.