Need advice

So my 18 month old daughter has a really hard time going #2. We do miralax every morning and suppositories when it's really bad. (she gets a suppository maybe twice a month) and 1 serving of cheese a day. When she finally has a bowel movement it's bigger than an adults and causes her tons of pain and discomfort. She's seen Dr's and been to the hospital and im doing everything they gave recommended. Could it be from her milk? Is due lactose intolerant maybe? It just breaks my heart to see such a little body go through and release something that an adult rarely goes through and she goes through it almost every time she goes #2.im at a loss and will try anything. TIA