I have a Kik profile and I am a 14 year old girl. Yes, i have gotten unwanted messages, inappropriate and otherwise. But on Kik, there is a "Block" feature, letting you stop any messages from someone you don't want to talk to.
There are parents who don't allow their children to have cell phones but they do let them have iPods, like I did when I was younger. Older iPods without the messaging feature can't text their other friends with iPhones or iPods, so Kik is a way for them to stay in touch.
There are also parents who ban social media like Instagram and Tumblr and Kik outright, and that just encourages their kid/s to use it behind their back. If all of their friends have it, they're going to want to have it, too.
In my opinion, Kik is an okay app as long as whoever's using it knows how to use it safely. If you teach your kids what to do in situations where they can be easily manipulated, they won't be as easily manipulated and might do what their parents taught them too.
Kids should not be allowed to use any social media or social anything site for any reason. Especially if their parents did not give permission. Kids don't know how responsible they have to be on the world-wide web, they might give information to strangers such as full names, phone numbers, addresses, etc. They won't know which parts of the sites are for adults, which could lead them to getting inappropriate images, videos, language or attitudes thrown at them.