I really wonder how Serena would fit into the Sun and Moon anime's atmosphere. Her having a crush on Ash really doesn't match the tone of that series.
Now that I think about it did the XY series say that Ash was 10 years old? I don't remember them saying that Ash was 10 in the first place since I always thought that the writers made us assume how old he is.
That being said. I think someone on this board had said that boys do not mature as much as the girls do in their early years. I'm paraphrasing a lot here but the reason why it was a one-sided crush was because Serena was at an early age to supposedly 'like' someone. Boys on the other hand don't really care about romance until they're in their adolescence years hence why Ash was oblivious to it.
I came late to the party but i can't recall them stating Ash's age. At least not as clearly as stating it in the official website. They probably approached it this way: Ash will be 10, 11, 12, 13, 14... whatever age the viewer decides he has based on his actions.
Perhaps they inserted amourshipping into the mix with a thought process of the sort: "10 year olds who watch the show will not notice Serena's crush on Ash at all while anyone older than 12/13 years old will easily identify the blatant hints. It's a win-win situation".
As for the crush being too one-sided, that's an argument that can be made regardless of the person(/pokémon?) being shipped with Ash. First of all what's important is that the other individual truly loves/has a crush on Ash, which Serena does. From there it's not so farfetch'd(...) to say that he likes her back, perhaps not in the form of constant blushing, but of inviting someone to travel with you, recognizing her worth, helping her find her dream, voluntarily giving her a ribbon, saying you'll always support her, the list goes on...
I do agree with Serena's crush not fitting the initial vibes of the Sun and Moon anime. All the more reason to want a resolution in the next canon 20 minutes.
Perhaps they're all actually 10 or 11, but have the mindsets of 14-16 year olds? It's not everyday that you see a 10 year old try to save the world by putting his life on the line multiple times, right?
But yeah, this, them traveling alone and all is why a romance restricted to a change of words(i like you...) and hugs/(cheek) kissing seems plausible and innocent. It's not like 10 year olds think they can just leave their houses once they're 10 and make their own pets fight on the streets, either. I mean, i knew i had some grasp of what fiction was when i was 10, notwithstanding the many times i threw invisible pokeballs in the air at home. Though i couldn't grasp some things and that's why at the time i thought Ash would inevitably always be older than me.