"weenie" is a term to talk about a penis in a harmless, almost innocent, "cutified" way, so you can mayvbe even talk to a young teenager about it. Did I get this right so far?
Yes, but the terminology is often linked to specific regions or ethnic backgrounds. A cute word used in California might be meaningless (or offensive) in Ohio. "Fanny" in the US refers to the buttocks, but in the UK it means female genitals. Someone who grew up in a Jewish neighborhood might understand vulgar Yiddish slang, but others might recognize only a few words (which they've learned from Mel Brooks films).
If so: is there an other term to used to talk about a female vagine in the same harmless, almost innocent, "cutified" way?
The USA is somewhat puritanical regarding public use of sexual language, to the point that a pair of legislators were
censured for saying "vagina" while debating reproductive issues.
I've seen parents use
silly nonsense words when talking to their daughters (with regard to bathing, toilet training, personal hygiene, etc). That's more of an adaptation than a deliberate euphemism, though. It's necessary to start explaining those issues
before the child has developed full language skills, so the child might simplify or mispronounce the word. The parents might then just accept the child's version of the word as canon, and continue to use it for years afterward (until they need to explain menstruation). Nonsense words wouldn't be useful in casual speech with other adults, but in the context of a sexual dialogue you could probably use random syllables and your audience will understand (
"I'm so horny! Please put your zim-zam in my fee-foo!").
I suppose that it depends on context. If you're trying to make a female character seem immature or uninformed about sexuality (such as a teenager's first sexual encounter) then it would probably be appropriate for her to use euphemisms. If you're writing dialogue for a normal adult woman who is speaking to an immature audience (such as a teacher seducing an underaged boy) then nonsense words might be more suitable.
Of course, it's important to remember that these are fantasy scenarios playing out inside a janky videogame. Most people
won't be trying to disbelieve or criticize your writing; they'll ignore minor problems and focus on the interesting parts. English is a weird mongrel language, and English speakers
expect to encounter new slang terms (for anatomy, positions, sex acts, etc) - especially in a sexual encounter with a stranger.
If you're very uncertain then you could always change the girl's background -- make her a German tourist, and then use your own sexual terms. If the English-speaking reader doesn't understand those words then it's okay, because the English-speaking
character isn't supposed to understand either :)