Hi
I'm so sorry for your losses. I can only imagine how gutted you must feel!
How old are they and were they vaccinated for Marek's Disease? The vaccine needs to be administered at hatch or in vitro and if you bought from a hatchery then you would need to specify that you wanted them vaccinated and pay a small premium.
Another possibility is coccidiosis. What does their poop look like? Age is also relevant in this respect.
The above are the two commonest killers of young birds and to be having several die in short succession it is likely to be one or the other. Worms will not kill several birds in such a short space of time.
You would be better getting come Corid (cattle section of your local farm store if you are in the USA, or look for a product that contains Amprolium if you live elsewhere) and treating them with that rather than trying to deworm them with Valbazen at this time.
There is no recognised treatment for Marek's, so you might as well treat for Coccidiosis and hope for the best. Treatment is 1.5 teaspoons of the powder or 2 teaspoons of the liquid Corid in a gallon of water made fresh each day for 5 days with no other water available. If they are not eating and drinking, as often happens when they are very sick with Coccidiosis, then a direct drench of the concentrated liquid of a couple of drops into their beak twice a day will help them fight it.
My gut feeling is that it is probably Marek's especially if they are adolescent birds at point of lay that were not vaccinated. Do they appear uncoordinated, like they cannot control their limbs or weak and lifeless. The former suggests Marek's, the latter Coccidiosis.
Are they keen to eat if you offer them food?
A photo of the sick bird and it's poop may help.
It would be best to get a necropsy done since you have lost more than one bird with the same symptoms. Where in the world are you located and we can help suggest a suitable lab that could perform such a service. Some states heavily subsidise these labs to protect the food chain, so it can be relatively inexpensive in some areas and it is import to know what you are dealing with when it is clearly a when you are suffering multiple losses.