Regular hatchlings, when inside an incubator, can't be directly selected, nor do they show status icons (like 'pending attack'). The only way to see the hatchling's status is to click on the incubator, then scroll down to its storage, and click on the critter in the 'storage' list. You can, however, still mark them for attack, and a dupe will come and attack them as expected, there's just no visual indicator that it's going to occur.
However, stone hatchlings seem to work fine, and can be selected just fine directly, and show status icons when marked for attack.
See screenshots for comparison. In both cases, the cursor is directly on top of the critter.
Hatch a hatchling in an incubator. Attempt to select the hatchling, observe that the hatchling cannot be interacted with directly.
Compare to a stone hatchling, which works fine.