Shape and proportions

I made myself as a 3D figure and it was quite easy with the help of some images on a flat surface and my shape is matched almost perfectly without any problems, but when I then would make a younger (around seven years old) version of myself I ran into problems, I began with "Kids 4" and moved the slider until the length math mode (I eyeballed it to 120 cm), but then nothing matched up; the head of the model was too large, the navel in the wrong place and so on, I was not able to make a realistic shape.

I do this because I have an idea for an animation in which a gray shape to pass between two pictures of a younger and an older version of me, so the proportion and lines is more importand than details and text use, I am only useing the navel and nipples to get the proportion to match up, but the actualy location are of no importance.

So I ran over to my friend who once several years ago toold me about Daz3D, he have a Genesis 3 with a "growing up" addon, but we got same problem and the model that suposed to be around six years old looked more like a shrunken adult than me as a child, after trying to adjust the mode it began to look really bad and the shape remended more about a sack of potatoes than me as a child.

Now my question is if there is a tutorial on how to fit the and shape models afrer a photo and how to make realistic models with realistic proportions?