This is a birthday present for LunaAlyxandra, who is awesome but who refused to tell me what pairing she wanted. I think you said you liked JamesxDom, so here you go, even if it's way too short.
When they were little, James at age six and Dom at seven, they were a team. Ginny and Bill had always been close and appreciated the two's friendship. They grew up, though, and were sick of the two adults constantly showing the pictures to everyone. It's one thing for your friends to see baby pictures of you (which is bad enough) and it's quite another to have your cousin (of the opposite gender) as part of the embarrassing act.
James is a fifth year now and Dommie's only a year ahead of him and it's not like they can really miss each other at Hogwarts (even if she is a Ravenclaw and he's a Gryffindor). They've been avoiding each other since they were eleven and twelve respectively and to be honest, it's getting a bit tedious. They're almost starting to forget why they hate each other in the first place.
James was almost scared to admit that he had a crush on her when they were little (she was a part-veela, but honestly, they're cousins- those sorts of things don't count as excuses) but he was a Gryffindor and he should be brave enough to tell her. Should be. Dom was way too afraid of what 'everyone would think' to let him know that she still liked him. There was a reason she was a Ravenclaw and she'd long suspected it wasn't for her intelligence. It was because she simply didn't fit anywhere else.
The grounds were glistening with the rain from the night before and when he found Dom, she was lying out over the damp grass with her hands behind her head. She ignored him, though, and just stared up at nothing.
He joined her, sitting on the wet grass, despite his better judgment and she stuck out her hand. He wrapped his pinky around hers like they use to when they were little, their silent promises. She sat up, pulling out a piece of paper. Without a word, she scrawled something across the parchment in her large, looping writing that he loved.
Forever? Friends forever?
He looked up at her and wrapped an arm around her petite figure, pulling her close.
"Forever."
"But… Hogwarts…"
He pulled the paper from her hands, writing his own message.
I love you.
She responded by slapping him across the face.
"Don't lie to me, James Potter!"
"I'm serious!"
She glanced at him for a few minutes before hooking her pinky around his with so much force that he let out a little screech.
"Promise? You're not just messing with me? You promise that you mean it?"
"I do, I do!"
They didn't close it with a kiss because a pinky swear was quite enough for them. Quite enough, indeed.