Hello there and welcome to BYC!
Sorry about your birds. Chickens that free range are going to be more susceptible to worms than run kept birds. DE will not take care of internal worms. It is great for external mites and such, but when DE gets wet, it loses it's ability to kill any internal worms.
So you might want to get them on a program of worming once or twice a year, depending on how badly they are picking up worms. You can take poop samples into your vet and for a small fee, they can test to see what type of worms you are dealing with and how often they are becoming infested.
Round worms are the most common of worm in poultry and do the most damage. Wazine (1 ounce per gallon of water) for 24 full hours will take care of all round worms. You will need to repeat this again in 10 to 12 days to get any eggs that have hatched as Wazine does not take care of eggs, only the live worms. (Eggs from these worms hatch after you worm, so this is why you will need to repeat this dosage)
Safeguard Liquid Goat Wormer is a great wormer and takes care of nearly all worms except a few species of Tapeworms. (Tapes are not all that common) 1/2 cc everyday for 3 days on standard sized fowl, 1/4 cc for bantam types every day for 3 days and they are wormed. I use either of these and have never had issues with worms.
Good luck with your flock. Worms can kill your birds over time, so it is important to keep them worm free. Here is an article from our learning center on internal worms and how damaging they can be...
https://www.backyardchickens.com/a/internal-parasites-parasitic-worms-in-chickens
If you have any further questions, feel free to ask.