When my daughter was in that age group, she really like having applique pieces on her clothing. I used nice bright colors and tried to use patterns of things she was "into" at the time. You could do anything --- flowers, birdhouses, baby animals, bees, gingerbread men, angels, etc. ---- I kept the appliques as simple as possible.
And instead of actually sewing them to the clothing, you might want to use the little velcro dots --- then the appliques can be changed to suit the mood of the day
Two sites that have some free applique patterns are:
www.freeapplique.com
http://quilting.about.com/library/0l...lks_appliq.htm
But if you do this search ---- (applique+free)
you will see a LOT of sites out there to pick from.
Another idea for tshirts is the iron-on transfers or tie-dying ---
Most of the free applique sites also have other ideas for embellishing garments.
Hope this helps some --- and have fun