I do personally think this is grooming... while there are no physical elements to the relationship, they're basically in mutual love and act/think based on it, which I think would very much shape a 10 y.o.'s mind while growing up. There's nothing sexual but it seems meant to be romantic, doesn't matter if it wasn't the older one starting it imo
Either way this is 100% fiction so eh, I'm not trying to start a riot ¯_(ツ)_/¯
More importantly, can we collectively appreciate the use of the Watersports
tag?