Woo Rin just continues to look for like a snobby jerk.
As for our MC, she's a sweetheart and her intentions are are lovely but naïve. As great as it is to make good memories, she really should have reported the dad. Someone doesn't just wake up one day and become physically abusive to their kids. Usually it's gradual, often starting with words, then maybe throwing objects, then it's a slap here and there and so on. If things have gotten to this point, these kids have likely been dealing with abuse for a while. Sadly too, once a person becomes like that, they are much more likely to continue to perpetuate or escalate the abuse without serious therapy or intervention. Plus she already knows grandma isn't able, or willing, to see what's really going on. Not to say that after reporting to the police everything will go smooth and easy, but saying nothing and leaving things as they are in the hopes that it won't happen again is a losing bet.