Just have Arlo do something other than feeding the chickens

Seriously. Dinosaur parent people Have your kid do something else. Clearly feeding the chickens is not his thing; not only is he afraid of it but his fear leads to him sucking at it, and that can only hurt your farm. Apply ten seconds of thought and give him a different chore.

And yes, eventually the dad gives him the job with Spot, but it should not have taken them as long as the movie implies to think of that, and the fact that the immediate thought was "Hm, better give him some other job that involves other creatures when he can barely handle freakin' insects!" doesn't imply either of them have anything close to common sense.