Is expiation obligatory upon the parents who caused the death of their daughter who drowned while swimming?

You are not responsible for what happened to the girl. Your going out for a picnic is permissible as long as there were no impediments, and you did not cause her death. Therefore, there is no sin upon you, nor any expiation. And say: "Indeed we belong to Allah, and indeed to Him we will return." (Quran 2:156). Caution and precaution for children are necessary in places where there is fear for them.