The slots could be marked in Jumbo Braille, Moon, and raised letters, depending on space. It is easier to feel than Braille. When touch is limited it is easy to cut oneself without knowing it. There should therefore be no sharp edges on the device. The input slot should have bevelled edges so that it is easy to insert the coins.
Dexterity can decrease with age.
With limited touch it is hard to tell if you have a good grip on something. The device should have a non-slip surface, perhaps ridged. A wrist strap would prevent it being dropped. If this were made of rubber it should be non-allergenic. Because it will be dropped, and because people with severe spasticity might dribble on it, the item should be waterproof and washable.