We are not familiar enough with this illness to know how the Individuals carriers in Montana will review it. If it is an illness that does not have widespread side-affects, they may offer you coverage with an elimination rider. This means that they may accept your youngest child, but eclude the treatment of pyloric stenosis from your covered benefits. The only way to know for sure is to apply with a couple of different insurance carriers in Montana. If you find out that she is uninsurable, you can obtain guaranteed issue coverage under the Montana Comprehensive Health Association. If your employer offers health insurance, you can also add your youngest child to your coverage. You do not need to add all of your children to your group plan.