Is there a custom that parents are alive and grandchildren erect monuments to grandparents?

Yes, but try not to be stood up by your grandchildren. The old man's son has the best monument. If his son doesn't, so can his daughter. It doesn't matter. You don't have to be superstitious. But remember, if you have a son, you should be erected by your son. Don't wait until Qingming. Before Qingming, just find a fortune teller to choose a day and go straight back to the grave.