I was sponsored as a dependent (my mom was the principal applicant and my brother was the sponsor). We submitted all of our passports at the same time but my mom and sister (also a dependent) landed months before I did. Don't quote me on this but I think you're supposed to submit all passports together since they'll all be given visas with the same validity (unless maybe they didn't get their medicals done at least the same time) or if you don't intend to sponsor your wife's son just yet. But if it's only going to be a short difference in the dates of their landing, I think everything should be submitted. By "until school is finished" do you mean the academic year or elementary school?