You are legally responsible for your child until they are 18. There is a universe of difference between not trusting your children vs taking reasonable steps to ensure their safety by knowing their location when necessary.
Parents have a legal right and obligation to know where their kids are. The technology used to achieve it is a mere implementation detail.
Also, technically, any adult who owns a smartphone has already consented to be permanently location tracked by their carrier and various entities that can buy location data from your carrier legally.
/s