Thank you, those are helpful thoughts.

It’s hard to know what to suggest without seeing them in action, especially since I haven’t taught first grade. I have, though, taught early childhood and a few things come to mind:
1. Rhythm, rhythm, rhythm. Hold them tightly so there is little to no time for unkindness.
2. Give them a reputation to live up to. Perhaps you can build in them an image of themselves as a kind and generous class. This works really well for individual children. You can say to another student, loud enough to be overheard, “Sally is so kind! Isn’t it great that if you’re looking for a friend you can always turn to Sally.” Or, “Sam is such a hard worker! I know that if I need something done in the room I can always ask Sam and he’ll do it really well.” Make sure you’re truthful and soon they’ll be appreciating each other in the same way.
3. Positive discipline. Compliment them a lot. Tell them how much you appreciate how kind they are to each other. Catch them in the act.
4. When someone is unkind make the whole world screech to a halt. Make a very big deal about it. Tell the class, “Oh, dear, so-and-so said something unkind! We never do that in our class. Let’s watch out for this and remind each other.”

As for social exercises, I wouldn’t really do these until after the 9 year change. The best for the younger grades is to build strong habits because they’re just not aware enough of each other yet (and they shouldn’t be woken up to this yet, either.) Keeping them dreamy and moving joyfully together is the best recipe for social harmony.