“Ladies nights” would seem to be illegal under Federal law, as sexual discrimination is banned by the Civil Rights Act, and it is not limited to one sex or the other.
I’m not familiar with the other promotion. If you mean they are charging children less, it could be justified if they can show there is a legitimate reason, like the children’s portions are smaller than the adult ones. If they charge children less than an adult for the same fare, then it’s age discrimination under Federal law.
Growing up I recall offers to charge children their weight for the kid’s meal. Different persons were assessed different prices for the same plate.