I have raised Heritage Turkeys for about 6 years. I have found that giving boiled eggs to poults under 4 to 6 weeks is a cause of pasty butts. I feed them a lot of boiled eggs but not until they are 4 to 6 weeks old for this reason!
The Marble Idea is a must as it will attract them to the water and also help keep them from drowning in it which will happen