A friend posted a photo of her daughters examining something tiny in the road. Up ahead of them is an enormous chasm where the road has been destroyed and washed out. They're absorbed in something minuscule while ignoring the most obvious point of interest. What idiom would describe this?
"He/She can't see the wood for the trees" is often used to describe a situation in which a person is so obsessed with the minor and insignificant details (the trees) that they can't see the bigger, and more important, picture (the wood)
I think you may consider:
not see further than one's (or the end of one's) nose
- Fail to consider different possibilities or to foresee the consequences of one’s actions.
If the insinuation is that they are purposefully ignoring the chasm:
the elephant in the room
"Elephant in the room" is an English metaphorical idiom for an obvious truth that is going unaddressed. The idiomatic expression also applies to an obvious problem or risk no one wants to discuss.
It is based on the idea/thought that an elephant in a room would be impossible to overlook. (source: Wikipedia)