The RGV is not known as a destination area. Don't know of any that take a plane there from up north to vacation, unless they have family. With that said...
If you're looking for something to do with a grandchild the zoo, butterfly garden, and bird sanctuary are all good ones. You also have movies at Sunrise mall. Maybe even a trip out to SPI to spend a day at the beach building sandcastles and/or taking a dolphin cruise. While on SPI there is a turtle sanctuary that would also be good for a grandchild. Don't know what the grand folks would like if they were alone though.