Who monitors kids' activity online?

I worry about the content my kids may watch online. I do not want to limit their access to breaking their freedom but I know the content they watch on Snapchat and other social networks. Are there any possible ways to see what they are looking at and maybe read their conversations with other people. I know that there are free Snapchat spy apps but I do not know how to use them as I am not a proficient mobile user. What can you recommend?

I don’t have kids, but don’t think spying on them will create a nice environment for the future.

Honest good/hard talk with them instead?
Using a little bit of content filtering?
Agreeing to having a shared account?