Plausible explanation.
But the movie still paused for me during the moment, meaning that I was no longer in the movie's world, but back to reality. After the scene, I was back in the fictional world, following the tale.
Those moments bothered me briefly at first, then a little bit more over the next couple of days, but it never took me out of enjoying the film.
I think the best explanation is that we were given ample evidence that she almost certainly received some training at a young age and was abandoned (most likely by Luke) to keep her hidden from the Knights of Ren.
Is Kylo supposed to be that much older than her, though? I suppose it is a decent age--let's say she is supposed to be 18/19, then he would need to be ~30 to make for a plausible enough gap in time for her to be abandoned/memory wiped at age 8 or 10. That's about how young she looked in the flashback; maybe younger?
But if she was hidden from Kylo and the Sith, would he not have any memory of her? Or did Luke train her away from the others and against their knowledge?
I was thinking that it's possible that she is Kylo's twin (no longer canon, but Han and Leia did have twins in those after-stories; still, this series is borrowing from previous canon.), but I think they are supposed to have a serious age gap. Or she was born later, and hidden away after Kylo went off for training. Anyway, that is all very implausible to me because I would think that neither Han nor Leia would have kept that from her in this episode.
Obi-wan's kid? maybe; but that requires a ton of back story and information that, if introduced or explained in the next two episodes, requires a lot of invention considering what is canon, no?