Constipation due to solids?

Our doctor said we need to up our daughter’s solid intake so she’s getting 2-3 ounces, 3 times a day. We’ve slowly increased to 2-3 ounces, twice a day from the once a day we were doing before. Of course, her stomach isn’t taking it that well even though we’re avoiding the more constipating foods and giving her pears with every meal. The doctor recommended prune juice, but wouldn’t give an amount, saying we should experiment. I’m frustrated because I see my daughter uncomfortable, particularly overnight and I have no idea how to help. How much prune juice should we start with? Are puréed prunes better?