A week without pooping is a dangerous situation for her to be in, her colon can stretch and possibly tear, a serious complication. I feel your frustration (and hers, poor dear.) Here are some tips:
Toddlers are notorious for having not-so-great diets that contribute to constipation. Go over hers with a fine-toothed comb and eliminate processed food, fatty foods, refined sugars and sweets, and bananas. Concentrate on fibrous fruits and vegetables, (prune, apple, pear, grape, broccoli, peas), beans, and whole-grain breads and cereals. Fiber Gummies help some, too, and she'll think she's getting a treat.
Increase her water consumption a lot, think water, water, WATER. Water down juices to get her to drink more if necessary. This will help the Miralax to work more efficiently as well.
Holding her poop is making her more constipated, though i understand why she's doing it, so encourage her to poop. If she's potting trained(ing) and I know it is difficult at best to poop train a constipated child, sit her on the potty at regular times each day to "try" and read her a story to keep her mind occupied and off her desire to "hold it in." If she's not trained lay her down and do "bicycle" motions with her legs a few times a day, and sit her in warm baths a couple of times a day to help her "release." (You can bleach the tub if she happens to go while in there.)
Increase her physical exercise. Toddlers need to be moving around (running, climbing, jumping) about an hour a day to "keep things moving," she may need more to get her system reset.
Here's a helpful article on constipation in little ones, cause and treatments:
http://children.webmd.com/toddler-constipation-causes-tre...
Liquid stool is actually a symptom of constipation. If she is having "drippy" poop and not the majority of it, it truly sounds as if the Miralax is trying to work, don't give up but call her doctor and ask about changing the dosage. Miralax is not a laxative to make her go, it is to add water to her poop to make it easier for her to go. Water is essential to help the Miralax to work, if she isn't drinking LOTS of water the Miralax cannot work, can't stress that enough.
Look into the Pedia-Lax laxatives for children ages 2-11. They work within 6 hours with no cramping and may be more comfortable for her than a suppository.
Work on treating the constipation and regulating her system with lifestyle changes at the same time will help in the long run. And do ask her doctor for a referral to a Pediatric Gastroenterologist who specializes in treating children with constipation.