This question already has an answer here:
The sentence in question: 子どもたちの表情は、いきいきと輝いていました
My attempt at translation: "The facial expression of the children shone in a lively way."
I wonder why と is used there? There is no reported speech or conditional clause involved. The only thing I can imagine is some connection to the "A が B と なる" function, because the facial expression "becomes" sparkling/shining when piggybackriding.
Or is いきいき and onomatopoeion and this is one of these cases where と just appears because of this onomatopoeion ?^^