N.
I guess I'm a little more forward when it comes to dealing with stuff like this 'cause I know I would have said something to boy #1 when he came back over to the house to play again. I would have said "It wasn't very nice for you to stop playing with son 1 name and son 2 name just so you could go jump on boy 2's trampoline. That really hurt son 1 (or 2?)'s feelings. I hope next time you have to make that choice, you will consider their feelings before you make your decision." It might not have accomplished anything as I know what you mean about why boy 1 did it, and he is only 6 years old, but I know that's how I would have responded, right or wrong.
Same thing with boy 2 probably. I'm very glad that you have taken it upon yourself to tell them to knock it off whenever they are saying mean things to your kids and you hear them. I think it's an adult's duty to correct a child when they are being mean to another. If we just sit by and let them do it, how will they ever learn that this behavior is unacceptable? I think inaction by an adult is perceived as a validation of misbehavior by a child. I understand your hesitancy to say anything to boy 2's parents though as it makes sense that you want to keep a neighborly relationship. I wouldn't make it a point to get the parents involved unless it's something more serious than the occasional mean remark.
It's a hard situation and maybe the best thing you can do is try to comfort your kids more when it happens than try to prevent it from happening to begin with. When my kids are feeling down, my husband and I try to divert them by offering to do something fun together. Redirection is always good for very young children when they are acting out or doing something they shouldn't. I think the same can apply to older children in some circumstances. For instance, if you see boy #1 go off with boy #2 again, you can tell your boys "Who wants to make cookies?" or do some other activity WITH them if that's the kind of thing they like to do. I know my kids LOVE it when my husband or I play with them. Whenever I notice that my daughter is sad for some reason, she instantly perks up whenever I offer to play barbies with her or play a board game or go outside and watch her jump on our trampoline or whatever. When I give her that extra attention when she feels rejected, it's like it takes away that bad feeling and she knows she is loved. I don't know if this works for all kids, but I know it definitely works for mine. If I can't beak away to "play" with them for some reason, I'll pull out an activity that they love, but that I don't let them do too often. I keep playdoh and paints put away for times when I want to keep my kids occupied with something else. If I say "Who wants to paint?", my kids go bonkers! My kids love to take a bath in our large jacuzzi tub, but we normally do morning showers. I can say "Who wants to take a bath?" and they are both running for the tub. Something so simple can really make their day.
Rejection is a part of life and we can't really protect our kids from it. But we can reassure our children that they are worthy and that they are special and that they are loved no matter what any other mean boy or girl says or does.
Blessings,
N.