Answered by Shaykh Ilyas Patel
Is it permissible for one to do the pelting of stones, during the hajj, on behalf of one’s parents, if one fears that they wont be able to do it themselves [due to the crowd]?

It is more virtuous for your parents to do the pelting of the stones during the night when there is fewer crowds, unless your parents are so weak or sick that they are unable to perform the salat standing up or they find it difficult to walk to the stones, then it would be permissible for you to do it on behalf of them.
– Ilyas