Jelly, 19 bytes

ḣ2ŒliⱮ=Ø.
ḲçƇ)hsQḣ7

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ḣ2ŒliⱮ=Ø. - Helper Link, valid word?: list of characters, Word; identifiers ("hs")
ḣ2 - head Word to index two - e.g. "Child" -> "Ch"
 Œl - lower-case -> X -> X = "ch"
 Ɱ - map across C in identifiers with:
 i - first (1-indexed) index of C in X -> [2,0]
 ('h' at index 2, no 's' exists)
 Ø. - [0,1]
 = - equal?
ḲçƇ)hsQḣ7 - Link get s-words: list of characters, T
Ḳ - split T at space characters -> Words
 )hs - set the right argument to "hs"
 Ƈ - filter keep those Words for which:
 ç - call the helper Link as a dyad - f(Word, "hs")
 Q - deduplicate
 ḣ7 - head to index seven

Jelly, 24 bytes

ḣ2ŒliⱮ=Ø.
ḲQçƇ)hsḣJf7ḢƊ$

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ḣ2ŒliⱮ=Ø. - Helper Link, valid word?: list of characters, Word; identifiers ("hs")
ḣ2 - head Word to index two - e.g. "Child" -> "Ch"
 Œl - lower-case -> X -> X = "ch"
 Ɱ - map across C in identifiers with:
 i - first (1-indexed) index of C in X -> [2,0]
 ('h' at index 2, no 's' exists)
 Ø. - [0,1]
 = - equal?
ḲQçƇ)hsḣJf7ḢƊ$ - Link get s-words: list of characters, T
Ḳ - split T at space characters -> Words
 Q  - deduplicate
 )hs - set the right argument to "hs"
 Ƈ - filter keep those Words for which:
 ç - call the helper Link as a dyad - f(Word, "hs")
 $ - last two links as a monad - f(ValidWords):
  Ɗ - last three links as a monad - g(ValidWords):
 J  - range of length -> [1,2,...,number of valid words]
 7 - seven
 f - filter-keep -> [7] or [] if less than seven valid words
 Ḣ - head -> 7 or 0
 ḣ - head of ValidWords to that index

Jelly, 19 bytes

^{Assuming that we must handle uppercase S and H too and that we must/may return the leftmost distinct "s-words"}

ḣ2ŒliⱮ=Ø.
ḲçƇ)hsQḣ7

A monadic Link that accepts a list of characters and yields a list of the words.

Try it online!

Jelly, 19 bytes

^{Assuming that we must handle uppercase S and H too and that we must/may return the leftmost distinct "s-words"}

ḣ2ŒliⱮ=Ø.
ḲçƇ)hsQḣ7

A monadic Link that accepts a list of characters and yields a list of the words.

Try it online!

How?

ḣ2ŒliⱮ=Ø. - Helper Link, valid word?: list of characters, Word; identifiers ("hs")
ḣ2 - head Word to index two - e.g. "Child" -> "Ch"
 Œl - lower-case -> X -> X = "ch"
 Ɱ - map across C in identifiers with:
 i - first (1-indexed) index of C in X -> [2,0]
 ('h' at index 2, no 's' exists)
 Ø. - [0,1]
 = - equal?
ḲçƇ)hsQḣ7 - Link get s-words: list of characters, T
Ḳ - split T at space characters -> Words
 )hs - set the right argument to "hs"
 Ƈ - filter keep those Words for which:
 ç - call the helper Link as a dyad - f(Word, "hs")
 Q - deduplicate
 ḣ7 - head to index seven

Jelly, 19 bytes

^{Assuming that we must handle uppercase S and H too and that we must/may return the leftmost distinct "s-words"}

Œlḣ2nTS=2
ḲçƇ)shQḣ7

A monadic Link that accepts a list of characters and yields a list of the words.

Try it online!

Jelly, 19 bytes

^{Assuming that we must handle uppercase S and H too and that we must/may return the leftmost distinct "s-words"}

ḣ2ŒliⱮ=Ø.
ḲçƇ)hsQḣ7

A monadic Link that accepts a list of characters and yields a list of the words.

Try it online!