Sew Many Designs - collection "Boytown USA" is a nice applique suitable on a toddler shirt.
Once you find the designs, check with the website owner to make sure you can use the designs for logos.
I did a google search (images) and found a lot of good designs. Quite a few were at Ann the Gran.
Just put this into a Google image search: Dump Truck Embroidery design
If you are using such designs on a regular basis why not get someone like basketkase to design some of your own pictures of dump trucks?