If the mother was healthy and she takes care of them, the babies should NOT be adversely affected at all. The health risks of them breeding so young is mainly to the mother. But, budgies in the wild do breed that young when conditions are right (they are adapted to breed quickly during the infrequent rainy seasons). The only likely issue would be that the new parents might not get it right and might not incubate or feed properly, resulting in stunted babies if you do not help out, but budgies are usually good parents. However, I might try the dummy eggs and replacing a few of the real eggs with fakes to make it easier on the new parents to care for the babies.
And, of course, provide lots of food, lots of their favorites as well as veggies, sprouted seed and other soft foods, egg food or hard boiled egg, and calcium sources.