Well they need to be hidden in plain site, start from the ground and only be as high as the oldest (or tallest) kid's eye site.
Here are some examples:
Low tree branches
Just tucked in the opening of a rain gutter drain pipe
Green ones in tuffts of grass
britghly colored ones by similarly colored flowers/plants
Hope this helps, my husband and I hid the ones for the family hunt last year. The kids ranged from 2-12 they all had fun. The last egg was hidden inside of my dad's gas grill(no it hadn't been used so it was not hot).