What you really need is to be able to send a signal direct into their hearing aids: an induction loop. I don't think there is any such thing.

If you want your keyboards to sound halfway decent you will not achieve that with a 6 inch speaker.

The fact that something will go loud does not mean that it has to go loud.   You should (IMHO) get a little suitcase rig. I like the Peavey a lot better than the Fender, and the Carlsbro one is not bad either.