Hi, OP
I would recommend trying the allnatural laxitives first (prunes, other?) before going the medicated route. Avoid stopping-up foods like, um, bananas? (There's got to be a list somewhere!)
And lots of comforting and praise for her trying.

Maybe also some distractions, like reading a book? (It' works for grownups *newspaperreadinghusbandimtalkingtoyou!*

)
She'll get the hang of it. And if it's really a terrible experience for her