Well, this may be because your server sucks. Have u ever thought of that? I have faith in you and your server so I'm going to give u some tips ok?
1) Be as friendly as possible, people who join need a good community.
2) Kids need quick and easy activities. Such as mini games like temple run or parkour. Kids have short attention spans so to make them play longer you need less time consuming activities. I see you have skyblock. Very time consuming. The thing with skyblock is that they need to be focused and be a huge fan of skyblock or else it wont make sence to them.. The problem with hunger games is that you need people to play with you or its not fun. Try plugins like mob arena or apocalypse based plugins. I have a zombie apocalypse mini-game and everyone who joins and plays it loves it!
3) Try asking what you can do for them. People like being told if they need anything to ask. When ever someone joins back say welcome back!
4) People always ask for op when they join my server for the first time. That's annoying but I found a fix. First u need a website with forums, much like enjin, or webs. They are free to make an serve full purpose. So anyways you tell them to apply for Admin/mod in my case I tell them to apply for jr.mod or Tec-builder so they can train for mod/head-builder. So step back and put into perspective how this process works. They make an app, and to do that they have to join your website, which in turn is joining the community. They spend time on your server making the app. Then when they are done, you tell them ok, I will check it soon. So they explore the server in the mean time, make new friends, join a faction maybe. And boom, you have them hooked. So in a couple hours or so you check his app. You give him the comment "This app was decent, but you need to play the server more to get to know all the staff, then I will consider your app." So he comes back the next day and plays more and gets woven into the community, he/she makes fourm posts helping out friends, or even considers donating to your server. Its a win/win for you server owns out there
5) Finally, check out other servers off of the updated list. It wont hurt to see what other servers that have players on use for plugins. TIP: if you have no acess to /plugins do /? same thing only not blocked. If you have any questions or you want to see my server just pm me anytime.
I hope this helps u guys out.