I update this app again. If you are in trouble that there is no suitable game for your elder parents, try Zealine.
My Mum likes it so much and push me to improve it. Its not that challenging that my Mum could play it everyday. She said this game makes her peaceful and relax, and it helps her spend boring time.
You just need move the balls with your finger, and no time limited. The rules are simple. Once you create a line of 5 balls with the same color in any direction, the balls will be removed. And you get score.
Believe me, your Mum and Dad will like it.