My son is two and takes singulair along with pulmicort and albuterol (when needed). He has been much more extreme in his behavior (throwing big fits where he just can't calm down and getting very upset over the littlest thing). I don't know if it's one, the other, or the combination of meds. He's on for asthma and allergies. If they need it for asthma control, I'd say try it and see. Not breathing is worse than some side effects. If they need it for allergies, I'd have them tested and see if you can control through other means and/or medicines. Good luck!