Yes, they could be. We don’t know. It’s possible their reasons/characters differ too.
I do think the isolation of the place they live has contributed. You need to be resourceful, you need to sort out your own stuff, you need to be independent, you need to get on with things and not make a fuss. Of course, drawing in vast search teams and the media to your land isn’t ’not making a fuss’ but maybe that wasn’t their choice. Who phoned the police? Was the delay in calling agreed by all three adults or did one adult want to phone them earlier?
Not really related, but I wonder if LE could offer some kind of deal to one or both grandparents if they reveal information about Gus’s whereabouts? Perhaps, indeed, the grandparents are planning to reveal his location either directly or by contriving to have him found, but are waiting until evidence of what caused his demise fades?