How Can FPIES In A Toddler Be Treated?

My granddaughter is suffering from FPIES. Her food diet is quite limited. She is almost 3 years old, and since late spring has been having many episodes of night terrors, especially at naptime. they can sometimes last up to 1 1/2 hrs, and quite violent. She has also started having real bad temper tantrums. A while back one of her doctors recommended Miralax to help her with constipation, due to her diet with FPIES. My daughter was very reluctant to give her this due to the ingredients in Miralax. The doctor belittled my daughter, saying her baby was suffering from the hard stools. So my daughter started giving my granddaughter Miralax on a very limited basis. She just related to me that when she has been taking this medicine, the terrors and tantrums seem to start. She is beside herself, thinking she may have caused her daughter more harm. Is there a possibility that the Miralax could be the cause of harmmlng this little girl or are related to her frightenly new issues that are upsetting the whole family? my daughter is a stay at home mom because of the childs many issues. my daughters life revolves around her 2 children. Please Help. thank you for listening. Carol C.