You can’t get away from the melodica being perceived as a toy by some, because they’re used by so many children, and come in a selection of bright colours!
Remember the harmonica (mouth organ) was initially a toy, which crossed over as musicians started taking it more seriously. So see yourself as a pioneer. An early adopter. It’s what you are.
Personally I don’t mind what people think. What really matters to me is what it sounds like. Like any instrument, if you put in the practice, it can sound beautiful. Just think what a violin sounds like played be a beginner. Most people are only used to hearing beginners on melodica.
Play them something great from YouTube!