"Don't harbor thoughts, master is still a child!" Did the boy also reincarnate from another world? I'm used to hearing that line in the reincarnation's head.
I think he only has feelings of love or admiration mistaken as love, nothing weird for a child liking another child. Even more normal when you consider that she shows him a little more grown-up side of her.
So get your minds out of the gutter folks ;-)
Given he's only 10, I'd say that "those thoughts" are still extremely pure. Probably limited to him fantasizing about them being married and/or wanting to hold her close. I mean, sure he might be aware of sex to an extent, or at least aware of what it means to be attracted to someone, but he's still quite young and has literally no experience(unless me has met a priest in the past. I hope not.). As @timesen said, get your heads out of the gutter. Wait for him to grow for 5 more years, then you can go swimming in the gutter all you want.