Is this okay?

So my baby is six weeks old and has constipation problems. We go to a really good doctor but I'm still paranoid because he said I could give her one ounce of water mixed with an ounce of prune or pineapple juice to help. I know I should trust him since he's the professional but I'm afraid to hurt her stomach. Has anyone tried this with the baby as young as mine? And did it work?