Understandable in this day and age.
There is no way to fully monitor your kid's activity on apps like Snapchat, Instagram, and Facebook other than logging into their accounts and viewing their accounts. Hopefully, you know your kids login info. If not, time to get it.
Regarding text messages, you would need to setup iMessage and FaceTime using your own Apple ID, but this would also mean they would have the ability to see your messages.
Another option would be to check their iPhones periodically. If you don't know their passcode, time to get it.
For location tracking, if the kids are using iPhones, you can set up Family Sharing. Check out this article on how to set it up and how it works: Family Sharing and location - Apple Support
Restrictions are also a parent's best friend. Check out how to set them up here: Use Restrictions on your iPhone, iPad, and iPod touch - Apple Support
So, basically, get all up in their business. Your house. Your rules. Right?