When she visits her future in-laws’ ranch in southern Texas, a headstrong city girl tries to save two illegal immigrant children when she discovers the ranch manager is using illegal immigrants for fertilizer.

Interesting idea. Curious to know if the in-laws kill migrant workers? If so, you could elevate the log line a little

'A headstrong young woman attempts to rescue two children from her prospective in-laws' Texan ranch when she discovers the family is killing undocumented migrant workers for use as fertilizer.'

Hope that helps.

Thanks.

A little cleanup for redundancy and marketability:
“When visiting future in-laws on a south Texas ranch, a headstrong woman must rescue two undocumented noncitizen children when she discovers the rancher is using migrant workers as fertilizer”

This logline is getting better, keep going!