at first i thought the choice was hard but actually if they are like 4 years old then its not that weird that each set of parents says to the other kid "actually, they're your real parents, and we're swapping you for our real kid". i don't think it would be that traumatic. it would take a while to adjust but eventually the parents have a kid they know is theirs and can easily raise it. they'd probably long term regret not swapping to their genetic kid.
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