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In dictionary,
If you say that something should have happened, you mean that it did not happed, but that you wish it had happened.
They wondered if they should ever have adopted a child at all.
( In this sentence, is it true that they didn't adopt a child,but they wish they had adopted a child?)
They wonder if they shouldn't have adopted a child at all.
(In this sentence, is it true that they adopted a child,but they wish they had not adopted a child?)
should have happened means that it did not happen, but that you wish it had happened.
EXAMPLES
(1) They're wondering if they should've ever adopted a child at all.
=> They adopted a child, but now they are reconsidering the choice they made.
(2) They're wondering if they shouldn't have adopted a child at all.
=> Same as (1)
should have 
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