Since you are new to the Wii, I thought I'd share this. Recently my kids are into creating different Mii characters. There are a whole bunch of videos on youtube for creating various TV characters using the standard tools that come with the Wii. Now when they play baseball they have Sonic, Master Chief, Spiderman etc... as the other field players.

Go to YouTube.com click on the Videos tab at the top of the home page and use "Mii Tutorials" as your search string. You will see a whole bunch of videos that walk you through creating various characters. Obviously depending on how old your kids are... you should probably be directly supervising them while accessing YouTube.