There are several secrets to making light dumplings. First, like biscuits, don't overmix the batter. Overmixing exercises the gluten in the flour and produces tough dumplings. In fact, some recipes call for the use of cake flour, which is lower in protein than all-purpose flour and therefore more likely to produce a tender dumpling.
A very important practice is to steam/simmer the dumplings. Keep the temperature of the stock to a low simmer. A rolling boil will cause the dumplings to break up. But more important, because most dumplings are bound together by eggs, you want to keep the temperature low enough to cook the dumplings without causing the proteins in the eggs to toughen up. Covering the pot will keep the dumplings surrounded by steam as they cook, allowing them to absorb the necessary liquid. It is so important to keep the steam in, in fact, that you should cover the pot with a glass pie-plate or other see-through lid so that you can see how the dumplings are cooking without having to take off the lid. Once they look fluffy, take off the lid and use a clean toothpick or skewer to see if they are done (as you would for a cake - if the toothpick comes out clean, the dumpling is done). Also, don't crowd the pan - the dumplings need room to expand.
The 1975 edition of Joy of Cooking (Canada, UK) offers this as the basic dumpling recipe: mix together a cup of cake flour, 2 teaspoons of double-acting baking powder, and 1/2-teaspoon of salt. In a measuring cup, break one egg and add enough milk to bring the volume up to 1/2 cup and blend. Stir the liquid into the dry ingredients; add more milk if necessary, but you want to keep a stiff batter. You can add a little chopped parsley, herbs, or onion, if you like, before mixing. Bring several cups of stock to a boil in a sauce pan, reduce to a simmer, and drop the batter by the spoonful into the stock. Cover and simmer for about 10 minutes. Serve at once.
And leave your mother alone - she's doing the best she can.