The general idea behind Frankie's appearance is he is a child in adult clothing, being unlikely that a child his age would be able to get hold of a suitably sized lab coat.
A rough example of the length of the lab coat on Frankie (although this sketch is not 'child-like' enough. He is sized more like a teenager and less of a child).
This is a side profile of how the lab coat will appear when he is walking along. Everything trails along behind him, much like the way 'Maggie Simpson' moves around.
In scenes where Frankie is operating something with his hands (which is the vast majority of the story), he will be seen with his sleeves rolled up, both because it gets the sleeves out of the way and generally makes him look like less of a 'ghost'. The cuteness/humour factor that comes from him dragging the coat along is a good thing, but it will outstay its welcome if this is overused.
Just a simple example of Frankie's facial appearance with his goggles worn on his head and being used. At the moment, he is looking very 'Calvin and Hobbes', but this appears to help emphasise the fact that he is a child, and it can also contribute to somewhat of a pun in which his goggles significantly magnify his eyes, giving him a more humorous appearance.
This is a comparison of height between Frankie and a typical adult, just to provide an idea of how small a child Frankie is. The 'Tom and Jerry' approach of adults being visible from the waist down alone is an idea which I may carry on into the final story, as it places the real focus on Frankie and the Creature.