Diets & Exercise: Infant, Miralax

How Much Miralax Should I Give My 6 Yo?

I have a stopped up little girl who is just recently out of pull-ups for BM's and is scared, therefore, resistant to "let it go" in the potty. Her Ped suggested Miralax, but I don't know how much to give her. I just bought a bottle at CVS and it says 17 grams for 16-adult. Anyone know how much to give?? Thanks everyone!