As a parent, it is your moral responsibility to protect your children from all the evil influences of the internet. For this, you can use the track a cell phone location for free Hoverwatch https://www.hoverwatch.com/track-phone-location
Of course, it is your responsibility as well as a duty to monitor and track your children's activity online and guide them properly. By this, you will be able to protect the kids from the harmful effects that they are regularly subject to when they’re online using their smartphones.
And, it is legal to use such programs. However, it is important to note that if your kids are teens, discuss the matter with them and let them know that you’re monitoring them and tracking their activity on the smartphone.