My son loves dinosaurs. For his 3rd birthday party we did a dinosaur theme. I had the cake made by someone, and the plastic dinosaurs and trees were a Target $1 spot find (great for party favor ideas as well). We had printed on the invitation for the kids to wear their camo to the birthday party to go along with the theme.
The t-shirts below were part of the party favors as well and the kids changed into them at the party. I downloaded some dinosaur clipart from the internet and added the kids names at the bottom (ex: Jaxon-O-Saurus) in MS Word. I grabbed a package of iron-on transfer paper and just printed the clipart and names on it. Iron them on a basic white t-shirt and you have custom shirts!