Kids are gifted. Mom never was but husband insists on her homeschooling them for religious reasons. Wrong?

Depends. I'm not a huge fan of home schooling, but data show that, effectively done, kids do as well as or better than those in public schools. There are many resources available, so one does not have to create an educational program from scratch. Biggest issues usually have to do with socialization, which can be harder; but, arranging for social contacts in other settings helps with that.
Safeguards. The age range for compulsory education is set by each state. During those years, school districts develop curricula with parents & monitor the academic progress of home-schooled children, using the same standards that exist for all children in the same grade. The legal consequences of not registering a child for school at the required age also vary by state & may be a jail sentence for parent.