I dont want to buy gummy worms or bugs. I want to make this as realistic as possible. He loves playing in mud and dirt, digging for crabs, bugs, worms, etc. I have some chocolate rocks I could spell out Happy birthday with on top.TIA>

How about molded chocolate or marzipan bugs?

finely crushed Oreos make great dirt!

that sounds like an awesome cake. how about making like a sand box type of cake; but adding dirt to it instead of sand , then adding all the little critters.

Heres a wonderful website you can get ideas from. I ♥ this webpage. ideas for the little critters that is.

http://www.marzipanworld.com/acatalog/Animal_Themes.html

This is a neat cake as well
http://www.cakeschocolatesandmore.com/about-press08-06.html

You could bake a 9x13 cake. Buy a pail & shovel, the ones the kids use for the beach. Turn the pail on it's side. Fill with cake. Place the pail on the cake by removing some of the cake so the pail can nestle comfortably.
I would ice the 9x13, & cake in the pail in dark chocolate icing. Crush some black & brown cookies. Then a liberal sprinkling of cookie crumbs cascading from the pail and on top of the cake should resemble "dirt'.
Since kids like gummie worms, you could place them coming out of the pail, and scattered on the cake. Perhaps make a few fondant catepillars, maybe a ladybug? They would look cute on the cake.
Hope these suggestions help

I'm not sure how many tiers you're going to make, but I think it would be really cute if you could make worm tunnels in the side of the cake, and put little gumpaste worms in the tunnels, and then decorate the top with grass and more dirt, and then put some gumpaste bugs on the top too. Kind of to make it look like an ant farm - you know how you can see the tunnels underground from the side view?? if you did chocolate buttercream you could cover it with crunched up oreos for dirt.

Thanks everyone for the ideas. I really like the sand pail idea. I'll post next week the final outcome.

Glad you like my sand pail idea. Isn't it great to go on a site like this and pick other decorator's brains!!!
I am not a "purist" when it comes to decorating, meaning, I don't believe that Everything on the cake has to be made of icing/gumpaste/fondant, etc.
If you use some store bought embellishments, and the cake looks great, and you achieve the look you want...then everyone is happy.
Good luck with the cake. Post pic's when you are done.