Very lovely and charming quest, thank you!
I noticed something I thought I'd mention in case it helps someone else: I did the quest once
and got one of the post pixie prizes
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the water statue
but then did it several more times and couldn't get the other post pixie prize
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the lemon tree
.Then I tried it one more time and tried clicking around more and got two new messages I hadn't seen before. The first was after building the barrel boat, a message about it being a great plant pot and taking it for later. The other was after the knight and princess are reunited, clicked on them and it said something about not disturbing them. After finishing that trip through the quest I got the other post pixie prize. So maybe one or both of those things might need to be done to get the other prize, I don't know. Just tossing that out there in case it helps!