Well, what does HE/SHE think they might like to play?
Have they asked to play something, or is it just you thinking they OUGHT to?
I have just replied on this topic elsewhere, but I will copy my reply in full, though it may not all be relevant in your case:
If you do a MN 'search' on my name, music, keyboard etc you will find I have replied several times to similar questions.
As a Teaching Assistant I taught recorder groups for ten years, had a Keyboard club for Yr6 children, for a while had a percussion club, and with Yr2 coached children to accompany the Christmas production each year.
Do you happen to know what sort of music activities the class has been doing?
Recorder is normally for Yr2 onwards, once fingers are wide enough to cover the holes accurately. Recorder is certainly the cheapest instrument, and an Aulos or Yamaha recorder is probably under £10.
Many schools have ukulele groups. Some schools do percussion - tuned percussion being xylophones, glockenspiel, etc; - untuned percussion is drums, tom toms, cymbals, etc.
But personally, if parents can afford it, I think an electronic Keyboard makes the best introduction to music for a young child. Ideally it needs to be 61 full-size keys, with a wide range of sounds, and plenty of rhythms and accompaniments. If it has MIDI (Musical instrument Digital Interface) it can be connected to a computer for recording or multi-tracking, but this is not essential. Some Keyboards will have their own built-in recording systems. Some also have, what they claim is an 'easy learning tuition system', but I am not keen on these, as learning 'properly' is not difficult.
There are numerous teach-yourself tutor books, often with a demonstration CD. Lessons are certainly not essential at first, as the tutor books have explanatory diagrams.