I agree with what the others have posted.
1) Some of the juvies may not be fully weaned or eating well enough to be removed from the relative safety of their parents' cage even at 3 weeks after fledging. Juvies that aren't fully weaned or eating well enough on their own are more at risk for "failure to thrive" when on their own. In other words, they're less likely to eat well enough to survive. It may take anywhere from a few days to a few weeks or even a couple of months before they may show outward symptoms of not thriving. However, if you were to hold them, you would be able to feel their keel bone and know they weren't putting on weight. Also, just because the parents may have started another clutch doesn't mean they are finished weaning/feeding the previous clutch. When the nodules on the sides of the juvie's beak have completely disappeared and the bird is no longer seen begging and you know for certain it is eating and drinking completely independently of the parents, then it is safe to remove them from the parents.
2) The juvies will establish a "pecking order" in their cage. As such, the weakest ones will be more at risk for being kept away from food/water. Multiple food/water sources may be needed in the cage to ensure that all of the birds are able to freely eat/drink.
3) You shouldn't need to use Scatt on the juvies if your parent birds are free from mites. Scatt is not recommended for use on birds younger than 6 months of age. If you are concerned about mites, you might want to consider using safer products -- i.e., Iverlux in the drinking water or Pestex (which is safe enough to spray on new hatchlings).
4) The humidity level of 50% could be causing coccidiosis to thrive and infect the birds. It also could be causing mold/mildew/fungus to thrive. All of which can be deadly to gouldians, especially young ones. Instead of keeping your bird area at a 50% humidity level, you might consider lowering the humidity to 40 - 43% and offering bath water once or twice a week.
5) Probiotics is not a preventative against disease or illness per se. It is used to help replenish "friendly" bacteria in the gut. Too much could cause "friendly" bacteria to over-populate the gut and cause the bird to be ill also. Adding Probiotics to the drinking water for one or two days after a group of juvies has been moved into the juvie cage would be okay, but it is not recommended to administer Probiotics on a regular basis.
6) There also may be a genetic factor at play. Some birds do not inherit a strong immune system. When they are subjected to stressors -- i.e., being removed from their parents, fighting for food/water, contending with cage bullies/aggression, subjected to environmental/dietary imbalances (such as high humidity and the Probiotics), etc. -- their immune system is unable to handle it and the bird may decline until the stressors are removed or it dies. Some die simply because they are not strong enough to survive even under proper conditions.
7) One other thing to consider is the diet. Birds, like all other animals - including humans, need a well-balanced nutritious diet to thrive. This means they need to consume sufficient quantities of protein, vitamins, minerals, amino acids, and essential fatty acids every day. The basics of protein, calcium, and some of the common vitamins are fairly easy to provide through fresh eggfood, eggshell, and a variety of fresh vegetables, but there are many nutrients they need that they won't be getting if that's all they're offered. Also, gouldians are known to be fairly picky eaters. And, in a flock environment, it may be hard to know if all of the birds are eating the food and not just some of them.
Also, a lot of folks fall into the trap of offering lots of green vegetables to their birds under the false assumption that it is healthy for the birds. However, quite a few dark green vegetables are high in iron and oxalic acid. Too much iron can induce iron toxicity in finches (causing liver problems) and oxalic acid binds to calcium and removes it from the bird. Oxalic acid also can cause stones to form in the kidneys. And, several green veggies are pretty much devoid of any nutritional value -- i.e., iceburg lettuce.
In general, it is wise to know the nutritional content of foods that you are considering offering to your birds -- to ensure that what you do offer is well-balanced and nutritious. Otherwise, it doesn't take long for finches (since they are so small) to become starved of nutrients.
The keys to ensuring strong, healthy gouldians are:
1) DRY and clean environment (cage/flight) and low overall humidity
2) clean water
3) good mix of fresh, clean seed (not just a mix of one or two millets and spray millet)
4) well-balanced, nutritious diet
5) lots of room to fly

Michele