I wouldn't just give sugar water (personally). I'd give them an electrolyte mix:
- 1 cup warm water
- 2 teaspoons molasses OR 2 teaspoons granulated sugar OR 2 teaspoons honey
- 1/8 teaspoon salt
- 1/8 teaspoon baking soda
- 1/8 teaspoon potassium chloride* (optional) (found in no-salt alternatives in grocery stores)
I hope your birds perk up soon!! I'm sorry you and your birds are dealing with this. I know it is very stressful.
Edit: I also agree with trying a food mash, or even some cooked scrambled egg to see if you can encourage an appetite.
If worst comes to worse, and they aren't drinking, using an eye dropper to squirt some fluids into their mouths may help. It's very easy to do, but easier with a second person. I always wrap the birds in a towel so they won't flap. I use this method often when giving orally injected medications.