My suggestion is that you can’t expect to make them happy. This is very difficult to do, and you might lose the chance to do other meaningful things and benefit other sentient beings.

Do what you can. Go to see them from time to time. When you are there, the best way to repay their kindness is to explain the Dharma to them, and encourage and inspire them to practice Dharma. Then, you will cause them to receive a good rebirth, because what they need is liberation from samsara. There is no future in just giving them mundane happiness. It doesn’t help them in the long run. You can try to make them happy, but you can’t expect them to be happy all the time. So, your main concern should be to repay their kindness in this way. Concentrate on putting effort into this for their sake.