I have my kids cell phones attached to my router. What can I install to view what they are browsing and the media they have? I want to make sure my kids are safe
One thing to realise is that you can't completely block what you kids do on the Internet and probably the best thing to do is to try and educate them on avoiding sites you think they shouldn't visit and also how to recognise dangerous situations (e.g. strangers looking to message them or meet up in real life).
That said there are products which can help to manage and provide basic restrictions on Internet access. They're not perfect but they can inhibit mild curiousity, and could help kids avoid "bait and switch" where a friend sends them a link which appears to go to a safe site but instead does not.
One thing you could look at is Family Shield from OpenDNS, which helps to filter access to adult sites, but there will be other similar products.
Rory's answer is spot on however, OpenDNS is not the only answer. Kids will be kids and they can become curious, some can become tinkerers looking to bypass OpenDNS. This is trivial for say a tween/teen. There are also parental apps for iPhone, and for Android. So you could go a step further and install one of those.
Education is always key and what I have done is, I take the time to speak to my tween at home letting him know that I will be checking on his usage by physically looking at his phone. This puts him on his best behavior as he knows that I will monitor what he does. I always take the time to explain why, and use real news articles to support my arguments. When a child sees the dangers versus what they may perceive as "mom/dad exaggerating" they are more prone to listen.