If you buy the expensive chocolate made out of pure cocoa that is something like 60% or 70% or more/higher in cocoa, there is no gluten and/or any milk in the chocolate.
So unless your daughter is actually allergic to chocolate itself, if you buy the dark chocolate, there is no milk product in the chocolate. Cocoa butter is not dairy. it's part of the cocoa plant/what is used to make the chocolate.
Pairing dark chocolate with peanut butter or flavored jelly like strawberry or raspberry or orange marmalade would encourage her to eat the dark chocolate (because I know most children prefer milk chocolate over dark).
The better, darker chocolate with the higher cocoa content can be found at Whole Foods, some Co-ops, Trader Joe's, and even HyVee. You need to read the ingredient list carefully and make sure there is no added milk/casein/whey (American chocolate companies like to add milk. Why, I don't know.) I eat dark chocolate because of a casein allergy and absolutely LOVE it!
If you would like some names of companies who make the dark chocolate without any milk whatsoever in them, let me know, and I can pass some names on to you.
You can also use Enjoy Life! chocolate chips, which have zero dairy/casein in them and are gluten free. Again, they are dark chocolate, not milk chocolate.
Also, CALM is a wonderful product to use with young children who have constipation. It comes in different flavors and is drunk in water. It is magnesium and allows the body to "naturally" eliminate itself. My 12-year-old daughter, who suffered from constipation terribly when she was younger (saw all the experts, used the miralax and all different types/sorts of meds), will take it if she hasn't gone in 2-3 days or she feels like she needs to go but can't. It's safe, simple, efficient, and all natural.