They do urinate, actually!
If you look closely (but not too closely) at a chicken's dropping, you should see that it is made up of three parts. All three parts are usually passed in one go after they are stored together in the cloaca.
The 'poo' bit is a brownish to greenish colour, depending on what you are feeding the chickens. The surrounding white bit is the uric acid part, as JH mentioned. It is made by the liver and is not soluble in water. It supposedly serves two main purposes. It requires less water to excrete than the water-soluble version, urea, that mammals make. Change in its colour can be a sign of disease. The third part is urine. It might be a little less obvious to see, but if you find a dropping on newspaper, you should see it as the watery "wet bit" around other 2 parts.
Hope that helps! It's so nice to see animals being part of the classroom again! When kids grow up with no idea where their food is coming from, or become overcome with fear or aggression - we've got something to be ashamed of... Just make sure they wash their hands afterwards! :)