Common Sense Media is a great website in general, and this article is great. I learned about Yik Yak from this article, and found out I never want my kids on it while they're still young. At least on my location (I live within 2 miles of three college campuses), it was all about sex and drinking and parties.
Other messaging and social media options might be more appropriate for your family. Read the article, download the apps and find out.
15 Sites and Apps Kids Are Heading to Beyond Facebook
I also found this article from Verizon to be informative, including information about game apps, too.
If you know more about apps kids are using, please share!