In the wild, Bearded Dragons get much of their hydration from the fruits and vegetables that they eat and from the ground they lay on by absorbing liquid through their vent area.
Hatchlings do not yet understand the concept of standing water in a bowl. They do not see it moving and it does not catch their interest. There are several things you can do to catch their attention;
One way to incite your hatchling to drink from a dish is to pour the water from apx. 6" above the dish so that the splashing noise and falling water will catch their attention. Splashing around in the water dish with your fingertips is also a good attention grabber.
A young bearded dragon should have water every other day.
You can put them in a clean bathtub or container with a few inches of luke warm water for apx 20 minutes.
A warm shower is always enjoyable as well.
An adult bearded dragon can be watered every other week.
Water is also know to stimulate a bowel movement.
If you suspect constipation at any time, A luke warm bath can be helpful.