I don't know anything about Benta Falls but I have been to YS Falls many times over the years and think you and your granddaughter might like it.
It's an all day trip and you may want to look into hiring a reliable driver to get you there and back. YS Falls has expanded thoughtfully over the years to provide all the facilities you might require. Each of three tiered falls has created a pool at its base. There is a track of graded
steps which allow you to walk to the top. The middle pool is great for swimming and there is a rope swing. The staff is stationed everywhere along the way to assist you and answer questions. Google "YS Falls" and check it out.