What you've described in your kids does not sound abnormal to me, although it sounds like they lack some self discipline (a skill that will come with time, if it's taught to them). In the scenario you described, for example, my 12-year-old daughter wouldn't have come back with the gallon of beans either -- and she's a model student, no behavior problems, on the honor roll with straight A's. I would have had to remind her at least once that the task at hand was bean-picking. It reminds me of one of my favorite poems:
I Meant To Do My Work Today
by Richard LeGallienne
I meant to do my work today,
But a brown bird sang in the apple tree,
And a butterfly flitted across the field,
And all the leaves were calling me.
And the wind went sighing over the land,
Tossing the grasses to and fro,
And a rainbow held out its shining hand,
So what could I do but laugh and go?
The point I'm trying to make is that everyone is distracted from work by what they would rather be doing. I don't think this is abnormal.
That being said, self-discipline is critical when it comes to success in school and life. Your son especially sounds like he may need some help in learning tips and tricks to make sure his work is done. Does he have a homework planner? If not, get him one. Have the teacher initial it each afternoon before he leaves school to ensure what he has on there is right, and you should initial each completed assignment in the planner (after ensuring it is actually complete, of course). This will help him get into the practice of writing down his assignments as they are assigned, keeping the planner with him at the end of the school day, checking off the list at home to make sure it's all done, etc. (My daughter's fourth grade class had to do something like this. I thought it was kind of annoying and too time consuming at first, but now that she's in 7th grade she's very good at using a homework planner and self-regulating due dates, etc. This is good because she's not one of those kids who easily keeps track of assignments and dates in her head.) Other tips and tricks may be needed to help him remember other things, like his gloves (what about the old string-across-the-back trick that keeps the gloves dangling outside the sleeves of the coat?). Or maybe a bedtime checklist on his wall (All the homework assignments in the backpack? Teeth brushed? Clothes out for the morning?) Maybe one week's completed checklist gets him a small reward, ten of them gets him something big. But remember that the key is to get him to remind himself of what needs to be done, not to rely on you to tell him to do it. The latter, in the long run, will not help.
Anyway, there's my two cents; I hope it helps. Your kids sound bright and sweet, I'm sure they will do well in life over the long term. Best of luck to your family. -LH