The sentence "Only children are not always selfish" is a misleading sentence to use as an example because it can have more than one meaning. It could imply adults are always selfish, for example, because it is solely [only] children who are not.
Sheep is another word where adding an S for plural is incorrect, it is both singular and plural. "I saw a sheep in one field and 12 sheep in the adjoining field".
These minor quirks are what makes the English language so rich, but also difficult to learn. Some cases have to be learnt individually, not by a hard and fast rule (its and it's for example).
Here's a little rhyme for your pupil :-
I take it you already know
Of tough and bough and cough and dough?
Others may stumble, but not you
On hiccough, thorough, slough, and through.
Well done and now you wish, perhaps
To learn of less familiar traps?
Beware of heard, a dreadful word
That looks like beard, and sounds like bird.
And dead: it's said like bed, not bead;
For goodness' sake, don't call it deed!
Watch out for meat and great and threat,
(They rhyme with suite and straight and debt.)
A moth is not a moth in mother
Nor both in bother, broth in brother.
And here is not a match for there,
Nor dear and fear, for bear and pear.
And then there's dose and rose and lose
Just look them up - and goose and choose.
And cork and work and card and ward
And font and front and word and sword
And do and go, then thwart and cart –
Come, come! I've hardly made a start.
A dreadful language? Why man alive!
I’d mastered it when I was five !
And yet to write it, the more I try,
I’ll not learn how ‘til the day I die.