It's because Dobby was a pure innocent. He didn't go into battle knowing he could die. He only wanted to help Harry. Also, as the below comment said- Fred died in the middle of battle. There wasn't time for gratuitous anguish. Same with Remus and Tonks.

Also children aren't my idea of a happy ending (don't want them), but they're a happy ending for these characters and that's all that matters. Because it's happy for them, I'm happy.

More importantly the series is in a huge part about love for family. It's what saved so many characters. And it's only appropriate that the flash forward with the characters involves that and those characters passing on that love to their own.