You're talking about building up immunity in chicks. It doesnt work like that. Chicks are more susceptible to diseases because their immune system isnt fully developed/functional. Case in point; coccidiosis. Even with chicks eating medicated feed can get coccidiosis, we see many threads about chicks with coccidiosis requiring corid or a sulfa drug, but very few with adult birds.I'm nit sure if this is worse or if it helps but when i get new chicks i usually try to bring them dirt,rocks, and grass from areas where my flock has been so if there is anything they carry they get exposed to it slowly instead of suddenly when they are adults after having no exposure to it.
Additionally if your hens had a disease, I guarantee your chicks would get infected as well and would most likely die or have to be culled. One exception might be worms because the chicks feet arnt touching soil. If they are, they should be wormed at 6-7 months old.