If they buy three regular one-day passes, they will not be able to park hop during any of those three days - they are One-day, One-park passes. The seasonal passes are such a good deal, especially if there is ANY chance they will go back, but you could compare the prices and decide. Just make sure that they will be valid the days you will be there, they might be blacked out thanksgiving week. Can't remember.
Just checked prices for you. Three one-day tickets will be $152.64 for each grandparent (50.88 per person, per day). A seasonal pass will be 189.74 and that is good in all of November (blackouts are Christmas, Easter and most of the summer.) A 4-day park hopper costs more - 203.52. You can buy any of these, except the one-day, one-parks if you have a DC membership. Sounds like the UPH or Seasonal pass would be the way to go if they want to park-hop.