Moonblaster + MIDI-Keyboard ?

You can use MIDI in of the Philips Music Module to play the instruments in the instrument library and that's it. No real time recording of notes.
But if you like to play keys on a superior keyboard and want to hear the MSX while doing so, it's an option

FAC Soundtracker pro has MIDI support as well but it's a very poor tracker and I wonder if anyone ever got something out of it using a MIDI keyboard.

The kids and me would not need a real time recording. It is just that they like doing some 'interesting' noises. Just as like on the real big Yamaha keybord of my father which had been sold for storage space.
It is not yet clear to me what Philips Music module you mean. Is there possibly some link or YT video to watch it in action ?
Which PSG do the Philips-MIDI capble modules use: Internal Yamaha-chip, external FM-Synth or better ?
Is a Carnivore or any other new cartridge supporting adhoc MIDI playback via software, not recording ?
Thanks so far.
Mark

MSXsynth is/was something really promissing to play chiptune in real time MIDI if I properly recall (PSG, SCC, perhaps FM in other versions in development).
But I don't know its status right now, the author has been quiet for a while... but it's already quite usable! Try it!