Yes that is true. To put it very simply and I haven't looked it up recently I think the numbers go something like this:
For Prednisone/Prednisolone I think it is somewhere about the 5 mg mark (adult). If taking more than 5mg then it is more likely that the adrenals will be shut down but factors such as size/weight and other things do come in to it as well. For Hydrocortisone about 20mgs (adult) and Dex goodness knows!
Maybe someone else can give you more specific information for children and Dex.
