From my perspective, foreign language acceleration can be done quite seamlessly, especially for a student with exposure to several languages. Each incremental language is easier to learn once you have a meta-structure of "how languages work". It is also relatively easier to accomplish if the languages studied are within the same linguistic family.
I had a secondary concentration in modern languages in my undergraduate, and was able to skip from a sophomore high school level in my third language to first year university studies for the subject major stream in that language (so a 2-year skip, more or less, based on a 4-year high school language sequence). The university department head granted me a discretionary set of credits on the basis of an in-office interview to fill the accreditation gap. (You might be able to accomplish similar permission for your DD/DS, especially given that they are "known quantities" at their school.)
It was relatively easy to self-study any gaps and build fluency in the 3rd language through participation in campus language clubs, chatting with foreign language speaking friends, and just immersing myself in media in the language. I took the full sequence of language courses for subject majors through undergraduate for my second and third languages, and a sophomore class for my fourth. (No acceleration in 2nd and 4th languages.) I didn't notice any persistent knowledge gaps as compared with the other language classes moving in lock-step. Achievement wasn't hurt at all by acceleration; I earned a scholarship for the top placement in the advanced upper year classes for the 3rd language major program. It's one of those subject areas where, once it's encoded into long term memory, it's there for good.
I would talk to the subject instructor to gauge the scope and sequence of the classes, and get an honest opinion of your DS/DD's abilities to determine the best course of action. Generally, language would be easier than literature, but there's no impediment to taking them as concurrent classes if your children have the inclination. Your decision(s) may be forced by scheduling considerations in other subjects. My guess is your children could easily absorb the SSA without batting an eyelash, especially if they love the language and have already had a few years of exposure to it. Then, they'd have additional scheduling capacity in upper years for other personal interest courses.
Which way are you leaning, currently?
