≫A child with an expressive disorder finds it difficult to express his/her thoughts... Co to znaczy w takim zdaniu finds it?
It means that this child "HAS A PROBLEM" expressing his thoughts because of the said disorder he’s suffering from.
Or "IT'S DIFFICULT" for such a child to express his thoughts because of the same reason.’
Or even :
‘TO EXPRESS HIS THOUGHTS' is difficult for a child with an expressive disorder.’
And now look what we can do with the latter sentence to generate back to your original one:
It’s all about movement of the constituents in a sentence.
So far “ to express his thoughts’ is the subject here , right ?
So we shift or move this subject to the end of the sentence employing a verb ‘find’ and a personal pronoun IT as a referring ‘prop’, which in fact will become the direct object of our generated sentence (your original one ):
“A child with an expressive disorder finds IT difficult to express his thoughts.”
Now , when we extraposed the subject " to express his thoughts', it became the 'to-infinitive 'complement of the object 'IT'.
The type is SVOCo ( Subject-Verb-Object-Complement)