This is a Yamaha P-32DP (or possibly a clone thereof). Coincidentally, I have one out by my desk that I acquired recently after having given my P-32D to my grandson. The little girl was probably stencilled on by a Japanese student. The P-32D is probably the most widely used melodica (keyboard harmonica) in Japanese schools. It is an excellent melodica, one of the best in my opinion, and it should clean up very nicely with a soft cloth dipped in rubbing alcohol. This can be applied to the keys as well as the rest of the exterior. The case can be scrubbed also with soapy water and a brush or dish scrubber. Be careful not to scrub off the little girl, though. Rinse thoroughly in warm water.
A three to one (or 2-1) mix of water and common distilled (unflavored) vinegar is the usual solution for washing the melodica, Hans. It eliminates odors as well as cleaning. Search here for “melodica bath” for more details. I normally soak the melodica for an hour or two in the solution, then blow it out several times, working the release button and all the keys. Then rinse very well and blow water through it a couple of times and dry out. Try it on the Yamaha first. This can be done without taking it apart, though I sometimes do take it apart to see whether further cleaning is advisable. But be careful not to bend the reeds or their gapping (space between the reed and the reed plate). I have not done this with button models, but it’s pretty safe with any model that does not contain wood (in which case I wouldn’t soak it). I also wouldn’t do it on a very old melodica with fragile deteriorating pads and such, for example old Italian models.