Weaning takes about two months of gradually reduced dosage. However, the adrenal glands are not permanently affected, and they eventually recover their normal function, although this may take many months. Given the length of treatment and generally young age, this is not seen as a risk. To be absolutely safe, the child must see an opthamologist at yearly intervals, which you should probably do anyway. For the most unlikely but worst possible outcome, cataracts can be corrected surgically.
Some children actually grow more during treatment.