My two year old daughter has been constipated since birth. You'd think after 2 yrs. of every method possible, we would have success & finally, we do. I wanted to start potty training her but about a month ago, she went 10 days w/o a bowel movement. I put her on Miralax & we've had GREAT success. Like you, my daughter is a great eater. She's almost 40 lbs. at 2 yrs. 9 mos. old! She'll eat veggies, oatmeal, apple juice, fruit, raisins, etc. & still can't go poo.
What happens is when she doesn't poo, the poo starts growing outward (horizontally) vs. vertical, the normal way. The poo then becomes too large to pass (thus brings blood & tears). Also, the dr. said it is a syndrome in which there isn't enough water carried into her bowels. The miralax helps do this. I will say she now successfully poos in the potty on a regular basis.
I know you may be skeptical of the word laxative but after 10 days of suppositories & everything else, we had to resort to some prevenative maintaince.
Hope to hear back from you. J.