Venī, venīte means "come" in both singular and plural. Perhaps "come one, come all" would be a good translation? Personally I would just have used venīte twice.
Spīritŭs means "spirit", while spīritūs means "spirits"; without the length marking on the u it could mean either. But the adjective sylvānī "of the forest" makes it clear that it's plural "spirits".
I'm guessing that dīvēs is a mistake. Literally it means something like "wealthy people"; a connection to dīvus "divine" exists, but is extremely old and doesn't really affect the meaning any more. Flūminārum is definitely a mistake: it means "of the rivers", but using the first declension ending on a third declension noun. It's like saying "childs" instead of "children" in English: it's easy to understand the meaning, but native speakers would consider it wrong.
As for the intended meaning, ignoring the mistakes, I'd say:
Come one, come all, spirits of the forest, divinities of the rivers.
(All the mistakes, by the way, are perfectly reasonable from a child who's just learning the language.)
EDIT: As Hugh pointed out in the comments, while sylvānus literally means "of the forest", it's most often used as the title of a particular deity. The more usual word for "of the forest" is sylvaticus.