What they should've done to make it more plausible
Many ppl complain about the mom being an idiot for sending her kids to meet with 2 grandparents they've never met, which is a total legit complaint because no Parent in their right mind would be this irresponsible. Here is what Shym should've done to make it more plausible:
1: should have shown the mom talking to the real grandparents on the phone, arranging the trip for the kids. We know this convo did in fact take place because the real grandparents were excited about the visit and that's how the lunatics found out.
2: should've shown the mom showing The kids an old ass picture of her parents from 20 yrs ago. Could even be a partially damaged one and her telling the kids something along these lines "I know this is an old picture, it's kind of blurry and ripped up a bit but it's the only pic I have of your grandparents since we haven't communicated in decades" - this way, it would be more reasonable for the kids to mistaken the identities given the only pic they had wasn't clear enough, plus one can argue ppl don't look the same after 20 years. kids probably think most elderly look similar to each other anyways
Had the writer/director bothered to add these 2 scenes in, then the story would be more plausible and the actions of the mom would be more justified, hence fewer complaints.