hello! your idea sounds really cool, and your question is super valid. opening scenes are really tricky, and i feel like you very rarely nail them on your first try. however, here’s a few things to keep in mind when writing your opening scene:
first, i like to look at plot structures that are used time and time again, and in this case, we’re going to look at the three-act plot structure. if you’re unfamiliar with it, there’s lots of good content on youtube about it and a bunch of articles online. so basically, with this structure, you start off with a ~taste~ of what we’re getting into, what your character is like before their life is completely uprooted. which, in your case, is her parents announcing their divorce. before we can get to that juicy stuff, though, you need to show us a few things.
- who is your character? what kind of person is she?
- why should i like her, and why should i care about her?
- establish the setting and give us a little bit of backstory
this last one is tricky because what tends to happen a lot is the dreaded info-dump 
, where you just pile all the information into the reader’s lap. which isn’t fun at all. you kind of have to find the balance between “what the heck is going on???” and “whoa hold on who’s this third cousin twice removed and why is there a cat?” so what you’re looking for, really, is the “ahhh, i’m picking up what you’re putting down and i want to read more.” that balance where the reader has questions, but not too many. feel free to leave a little up in the air–you have an entire novel to develop answers to these questions. your readers can pick up on little things and they can just Not Know. that’s what makes books interesting, the not knowing.
so, establish where we are. is this taking place in a mythology-inspired british regency novel with dwarves, or present-day turkey?
establish your main character. who is she? why should i care about her?
then, set up the skeleton of your plot. your opening scene doesn’t have to dive head-first into plot, but it shouldn’t be irrelevant to the story either. what does her everyday life look like while also giving me a basis of what this book is going to be about. if it’s about a zombie-hunter in training, then show me what a training looks like. give me a little of foreshadowing of what’s to come before we reach the inciting incident (when their life is uprooted–again, this is three-act plot structure! super cool!).
one of my favorite authors wrote this thingy once about how she makes a three-chapter rule. if the plot doesn’t show up before chapter three, then some rewriting needs to happen. that might be something you stick with, and it’s something i find useful too. also!! remember!! this does not, in any way have to be perfect first try!! as a matter of fact, it probably won’t be perfect first try. use it to figure out your character, figure out your world and get something down. you can’t figure out if it’s right until you’ve written something wrong.
so, with this being said, looking at your options, it really depends. which one feels the most engaging? which one is going to show us the most of your MC? go with that one. if neither of them fit, then it doesn’t hurt to brainstorm something completely different! if you’re going with option A (her reminiscing on when her parents told her they were getting a divorce and then her arriving at her grandparents), then make sure it doesn’t get to info-dumpy. gradually weave in the divorce. if you’re going with option B (start it when her parents told her they were getting a divorce) then make sure you’re showing us a good taste of who the main character is and also make sure it’s not too info-dumpy. opening scenes can be an information overload!
so uh, this was a lot of information! i cannot stress enough that it doesn’t have to be perfect on take one, you just need editing and time. don’t be too hard on yourself. happy writing! also if i wasn’t clear on anything/you have questions, feel free to ask!
)
[name_f]My[/name_f] Dearest Diary-Journal-Thing That I Keep On [name_f]My[/name_f] Phone, 
