How can I make these bath valves easier to use?
As shoemaker said lever handles are a lot easier for the elderly to use, you may want to consider changing to ceramic disk lever handle taps( the whole top assembly) as this don't get harder to turn off as the washer wears out because they don't have one. They have a ceramic disk which usually last about 3-5 years before needing replacement. 

If you decide to keep the old taps you would want to disassemble the "shaft" or spindle and replace the washers in it (jumper valve and spindle o-ring/s), clean the thread inside the spindle housing and put some tap grease in there to make it easier to use.

This link shows the tool for cleaning the inside of the spindle and you could also reseat the brass that the washer seats against with a reseating tool p.s. I wouldn't recommending using the one pictured as they are rubbish and leave a ridge In the seat but it will do for illustration purposes.

Then make sure what type of handle you need the porcline lever handle set in the picture has a centre external screw, which doesn't look like it will fit with your spindles as they all have an external nut to hold the handle on, but just check the old ones before purchasing new ones to save wasting money on taps you can't use.