Miralax is mild enough to use in children, but you should be careful with it. I had Hyperemesis Gravidarum when I was pregnant and the only thing that stopped the sickness was Zofran- which came with the side effect of constipation. Not normal constipation either. I was admitted to the hospital 3 times during the pregnancy for bowel obstructions. The pain was unimaginable, and I had to stay there until I ...relieved myself.
One thing that they did for me once things would kind of start moving again was give me lots of yogurt. Activia has the highest amounts of probiotics and it's very tastey too. That will put the good bacteria back in her tummy and help her stomach and bowels to do what they're supposed to do. I said be careful with Miralax because like with any laxative agent, your body can get dependent on it and basically your intestines like any muscle, atrophy and you can't poop unless you use the meds. That's a bad place to be in too. But that's far away from where she is now.
Sounds like her diet isn't working for her right now. She could have some undiscovered food allergy that's causing stomach problems or digestion upset. I second the person who said perhaps she should see a pediatric gastroenterologist.
Other things that worked for me when I was in a bad bad place were:
Broccoli, Mini Wheats cereal, Cheerios cereal, hot oatmeal, nuts, hot or warm drinks like tea or coffee (I know she's a kiddo, but apple cider works great too).
Good luck mama and don't stop asking questions until they help her!