Hi I think the short answer is that, in informal and lyrical English, "There's" is allowed before a plural because... (a) it just is very common in casual, spoken English (b) it is especially allowed if a single syllable is needed in the song or poem. (otherwise, "There're" would need two syllables) This question appeared in EnglishForward nearly eight years ago!
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.