A plot bunny looking for input

[Bob Schroeck]

(I recall mention of a mother whose baby carriage rolls away while she is conversing with a neighbor), and that they do grow to independent childhood (since there are no infants among the Lost Boys we see)

If I'm not mistaken, that's Peter himself.

they would be forced to agree to his returning to Neverland over the summer...

Presumably because Peter forced the agreement somehow, and because the Dursleys simply aren't in the picture any more -- the blood wards certainly have decayed to nothing in the ensuing years, so there's no point -- except for a Manipulative!Dumbledore's typical plot to keep Harry downtrodden -- in putting him with them anyway. Although it would be interesting to see the wizards decide they don't need to abide by the agreement because they figure a boy in Neverland can't do anything to them if they abrogate it, only to find out they're very wrong...

he really shouldn't be older than about six

Physically. Chronologically, though... but then you note that already in your bunny text.

part of the idea is that Harry would in fact come to the wizarding world with the mindset, worldview and general background of a Lost Boy

I am so anticipating the first time he crows in the Great Hall.

Would he even be allowed to play for a House team, given his physical age?

Would he be disqualified for not needing a broom?

and the likelihood that the wizards who had made the visit and discovered the existence of Neverland would themselves want to learn more about it; interesting fallout could arise from any of those.

I could see one or more Wizarding factions deciding to exploit Neverland in some way... which could lead to an interesting coalition of "native" forces united to repel the invaders...

I don't think Tinkerbell as a house elf works

Of course not.  She a non-Cornish variety of pixie, obviously.
-- Bob
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Then the horns kicked in...
...and my shoes began to squeak.