A: it's a human whose breasts I'm complaining about. B: it's not an attractive shape. It's really wierd. They could be the shape of the Washington Monument, but I wouldn't be a fan of that either. If the reason for the shape is because the character got a boobjob, then it's plot and character at the cost of attractiveness. Here it's not.
In the US, 10 years old is elementary school. I remember a bit about elementary school. Yes, people had crushes, often foolishly mistaking them for love, but they didn't last 10 months, let alone 10 years. 10 year olds can have affections that they perceive as strong, and humans can remain in love for many years, but 10 year old humans can't do both at the same time. My experiences tell me that that is not plausible.
As a result, it makes the character unbelievable, thus disconnecting them from the audience. That is too unrealistic, even for fantasy, since it harms the viewer's experience.
As I said, "there are plenty of other ways for them to get set on a romantic path," and this one, which seems completely implausible to me, is thus quite poor.