You're off to a great start! The most important thing is to have fun with it and striving to get better. A few thoughts/notes/comments that may help:
- On the photos you posted, the focus does look soft. Your camera/lens choice is fine, but check your focus settings.
- Don't be afraid to use a flash in full daylight for the close-up (non-game) shots. It can help fill in the shadows under the brim of the hat and around the eyes. If you're not used to this, check it out.
For game shots,
- Try to find a good backdrop to shoot against. What's behind the players will make a huge difference. It's not always easy and you don't get to choose the field, but try to find an angle that will keep the focus of attention on the player and not the objects behind him.
- I like the way you're shooting at eye-level with the kids. Many adults shoot down at them. It's a + to be on their level!
- Spend some time browsing through other people's images and Google images for ideas on shots, framing, exposures, etc.
Just my 2¢. Thanks for sharing your pics!