Constipation is common in toddlers as they are learning to toilet train etc. When my daughter went through it at this age as well, we were referred to a pediatric Gastroenterologist.
We were told that constipation like this is two-fold issue... it is physical/biological and emotional. For your daughter, since she eats well and healthily... you may want to increase her liquid intake... water, pear juice, prune juice, "whole" apple juice, which is what was recommended to us. Certain foods are "binding" and certain foods are not. So feed her liquids/foods which will promote a looser stool... (sorry I don't remember all of them).
Keep in mind that overcoming it, the constipation, can take from a month to several months. As the child gets more and more constipated (by "with-holding" it), the stool gets more and more impacted in their system....then this makes it more and more difficult to pass stool, and it makes it more and more painful to go to the toilet to let it out. It becomes a vicious cycle.... then, because of that, the child "withholds" even more.
For us, in conjunction with a good fiber diet and liquids, we were told to give our girl Mineral oil. (I forget the quantity)... but, this will internally loosen the stool and soften it and make it easier to release it, diminishing the associated "pain" from constipation. I would not overdo laxatives/suppositories etc. on a toddler. I do not know about Miralax either as we never used that.
You may also try to try Homeopathic alternatives. I'm sure if you do a Google search there will probably be things that pop up.
This is just my experience and do not intend for this to be a "prescription" for your girl. However, in our case, it took about 1-2 months for my daughter to get over the constipation problem. If your regime for your daughter is not working... I would recommend seeing a specialist. Remember, that as the child withholds from pooping and gets more constipated... the system gets more impacted and it just exacerbates the problem.
Just wanted to share our experience.... Good luck, I know it's not comfortable, and take care.
~Susan
www..cafepress.com/littlegoogoo