Trying to fix this for kids, can't log on using other computers

[pikachu784]

If your hosting this at your home, it's most likely from the router firewall. You need to port forward the server. This depends on your ISP and your router type. Look up how to port forward on your router and ISP.

This is most likely the problem, I had the same problem. Also check what your public IP is, you may be trying to connect your your private IP.

Also, update frequently, MCServer is continuously getting needed updates that should fix many parts of the server.

EDIT: This is definitely because people on other networks are trying to connect to your private IP. Check http://www.whatsmyip.org/ to find out your public IP or just search Google.