At junior school we had melodicas - the plain, green-and-white, two octave Hohner model, and there was a melodica orchestra. They were not so costly those days, and that is why schools were using them - we had to buy our own (mine has been consigned to some rubbish-heap some decades ago, I am sorry to say). It might have been something to do with the memories of the cacophony produced by dozens of little fingers and little lungs.

However, there is no such thing as a "silly" or "childish" instrument - they are all serious. The question is rather where the sound of melodica would fit. I would not equate it to either harmonica or melodeon/accordeon, but more to those small foot-pumped church organs. Sensitively applied, it could enhance 5-10% of songs or tunes. But used indisciminately it will "grate" in my ears.

I also remember two limitations of the melodica: It was impossible to play really softly - the reeds would only start vibrating with a fairly heafty blow down the mouthpiece. And the keys had veeery slooow action, making it difficult to use for really fast tunes.

But if a slow, loud tune is your thing - like a fanfare, say - then melodicas would be fine.