In general, it goes with the next word. If it links clauses, for example, with perhaps some exceptions in poetry (and some odd usages in Plautus), it goes on the first word of the clause.
The one exception to this rule you'll likely encounter with some frequency is when the next word is a preposition, and then it often (but not always) goes with the next substantive. Lewis and Short gives a few examples:
VII. Que is usually appended to the first word of the phrase, but to a noun rather than to a monosyllabic preposition governing it, unless the preposition is repeated: “de provinciāque,” Cic. Verr. 2, 2, 20, § 48: “per vimque,” id. Phil. 5, 4, 10; cf.: “ab iisque,” id. Tusc. 5, 33, 94: “sub occasumque solis,” Caes. B. G. 2, 11.— “Exceptions are to be found, especially in Liv.: proque ignoto,” Plaut. Capt. 3, 4, 10: “exque eo tempore,” Cic. Off. 2, 23, 80; 1, 34, 122: “inque eam rem,” id. Rosc. Am. 39, 114: “inque eo exercitu,” id. Sest. 18, 41
So, with the words you provided, puella should have the -que. However, you don't really need to repeat parv-. If the two are a simple subject, you can just as easily write:
puer puellaque sunt parvi or puer puellaque sunt parvae
Depending on the rest of the sentence, you could even have something like:
parvus puer puellaque
But repeating parvus means you're placing some sort of emphasis on the word (which is also fine, if that's what you're consciously doing).