We've been through this & then some over the years. Mine ended up with a diagnosis of Ehlers Danlos Hypermobility & the blockages were down to an extra stretchy guy, that plus a soy allergy, so maybe
Apple juice is a natural laxative, so that would be the best to give her & probably why the doctor has suggested juice ??
You need to cut out all white carbs, no bread, no pasta etc - these all slow the transit of food through the gut right down. Until the constipation is clear if you can keep her on soft fruits, this will,help with liquid & roughage
How about getting her making smoothies with you, lots if fruit, lots of apple juice & sneak some movicol into the mix, or better still let her have some control & add it herself & have her drink those.
First off though, get on the internet & find child friendly explanations of a drip & explain to her that if she doesn't drink, then that's what the doctors will have to do. Then suggest the smoothie making & offer her seething to collect, that you give her for each drink she has - we used pennies painted with bail.polish & called them flower coins - she collected 10 & she got a trip to the pound shop & a £1 to spend
Good luck