does 'sina' here indicate the plural of the parents or the plural of the children?
plural of children, it would be Han tackar sina barn if it were only one person as subject.
That's the trick ... pay attention to the adjective with ett words, it will tell you if the noun is singular or plural. :) I am improving!!!
so, how would the sentence be written if it translated as 'The parents thank their child'? What would sina be changed to?
Föräldrarna tackar sitt barn.
Barn is an ett-noun. For an en-noun, e.g. "son", it would be "Föräldrarna tackar sin son".