If a snake is eating a human infant in front of me what should I do? What is the difference if the snake was eating a frog or a human? What do Buddhist scriptures say about this?
Just kill the snake and save the baby

Ok , this would be the reaction of a person who has no idea what Buddhism is . But , even if we know Buddhism to some extent, we would do the same . Because still our minds are not developed. 

According to Buddhism 

Basically all lives are equal , if a snake is eating a frog , there will be no reaction from a person who has achieved nirvana. If the snake is eating a human (even if the infant is his own kid , there won't be a difference )he would never kill the snake , he will try to remove the snake using a stick , if it doesn't succeed,he will give himself to the snake and will save the life of his kid or any other human. But if the snake is eating some other animal, he will not interfere . Because according do Buddhism , people should not interfere with animal lives , all those things are unnecessary attachments. Buddha advises not to have pets , not to even give food to animals continuously , because if you do those animals will start to make bonds with you and you with them , that will definitely make ones samsara longer . 

Conclusion

People should not interfere with actions of animals. But you can save any human being by sacrificing yourself or by using some strategy