well yeah clothes - with loads of pockets for the things they collect on walks, sun hats, book bags would be awesome!! What about dress up type outfits? Army, Fireman, Chef, Wizard etc (the girls would go for the dress up things too... it's a big deal with the 3 - 9 age groups)
More for boys that would be a hit:
dungarees, t-shirts (like the layered look, where it looks like one shirt under the other, and plain shirts too), long shorts in denim, corduroy, cotton - all darker colours, maybe you can find a stain resistant fabric for some of the pants.
Accessories: sun hats, book bags, swag bag/treasure bag for carrying around their odds and ends...
