V.B.
How old is your youngest with the sneezing issue? My son was about 4 years old when he began sneezing regularly. And it wasn't just A sneeze at predictable times of day. It was repetitive sneezing (like 4 or 5 sneezes in rapid succession) a few times a day.
His preschool teacher even noticed and asked if he could be allergic to anything in her room.
Fast forward to about age 7 and son was diagnoses with allergic rhinitis (season allergies) that were severe enough that he had "the tell tale wrinkle across his nose" from where he would wipe with his hand. He was prescribed Singulair, but the tablets gave him stomach aches about 30 minutes after he took them, so we discontinued use. Then he was prescribed a nasal spray (I think Nasonex) and it seemed to help a little, but not 100% effective, even combined with OTC antihistamines. Then his doctor closed his practice to move out of state (he was overwhelmed running a private, single physician practice, and moved to be nearer his own extended family and be in a group practice so he actually had some family time). It took us a while to find another family physician we were happy with. By then, it had become such a normal part of life that I just bought the bulk boxes of Claritin or whatever and went on with life.
Then we looked into immunotherapy. At age 11 (almost 12) son started getting allergy injections to combat his "season" allergies (that were continual from one season to the next with various different allergens, so that he had symptoms all year round except for about 2 weeks in December... literally). They did a blood panel and turns out he did in fact have allergens he was reactive to that occurred all throughout the year, except there were a few weeks in December that nothing was in bloom or pollinating, or whatever. Those 2 weeks were the only relief he got. OTC stuff did nothing by then. And sometimes, he would have a cold the 2 weeks he didn't have allergies happening.
It took a few years to reach the effective/maintenance dose with his shots, but now, at 17, he is virtually free of all symptoms (unless he is in close contact with a cat.. this allergy developed in high school and wasn't part of the panel of allergens he was treated for). I no longer need stock in Kleenex or Puffs. And I occasionally notice that we don't have an Allegra or Zyrtec in the house!
All that to say, it may be that your youngest has allergies developing. Age 4 is about when it started with my son. So, be aware that that is what it might be, and if it turns out to be the case, consider testing and immunotherapy. According to our doctor, it is most effective when done before puberty. After that, early adulthood or adults do not have the same effectiveness rate with the treatments, AND they treatment time lasts much longer. For my pre-pubescent son it was 4.5 years. For adults, it can be 6 years.
But... it worked. And I feel terrible when I think back and realize that we might could have treated his allergies sooner and he might have had relief from that all much sooner. Not only did it affect him during the day, but he finally was able to get a restful night's sleep! Which is crucial for growing kids. :/