Before discovering my son was celiac i was told he was small due to a "too healthy diet", which is rubbish because he eats all sorts of foods. He has been gluten free for a year and is smaller than my 4 year old even though he is also 6. I have tried a high fat diet, but found it is really easy to get into bad eating habits. I found myself giving him extra serves of ice-cream etc. Its hard because a lot of foods that are high in fat are also high in sugar and salt. I have toned it down now and i just don't think this is healthy for a young child no matter how small they are. My current doctor suggested to put a slice of butter on top of every meal which works because you are raising the calories without increasing the junk.