Absolutely agree with other posters about teaching children that all behaviours have consequences. Some behaviours result in nice consequences and some behaviours result in not so nice consequences.
Even at 2 and 4 your grandsons are old enough to understand that if they continue to do X then Y will follow. Tell them clearly what will happen and then implement it. As others have said, no ifs, no buts, just do it. Tantrums and tears will result in time out in their room. Quietly pick them up, take them to their room and tell them when they can behave properly and put on a happy face they can come out.
Only you and the parents know what will have an impact on these two little boys whether it is removing favourite toys or TV shows for a certain period, no favourite foods, special outings etc.
Good luck but it's worth it in the long run.
Angela Rayner lashes out and calls Sunak “pint sized loser”.