Movicol is one of the bect constipation meds available - if not the best. That said its not going to suit everyone.
I expect your consultant explained how and why it is different to other constipation meds like lactulose and senna.
My DD is 11 and never drank much and suffers with constipation and tummy pain issues. The movicol is working OK for her atm but not 100% but alot better than fybogel, lactulose, senna and loads of other meds we have tried.
I am on adult movicol long term for similar issues. Normal foods that make people go to the loo just have zero effect on me and my DD - rhubarb, prunes, apple, stewed fruits. I also have spasms so bad I can pass out. Picolax has even failed on me - well 3 doses and lots of enemas before things moved
. Movicol is OK - not without a few problems but works OK.
My DD never feels thirsty so wont drink unprompted and struggles to drink much at all because of this. She gets her fluid from smoothies, ice lollies (home made with juice/sugar free squash), high water fruit and veg like chopped up melon, salads etc. Obviously we encourage her to drink as well.
Perhaps you need to go back to the Drs and discuss the options because her diet and digestive system will have changed abd developed alot in the last 6 months and she may tolerate and cope wit movicol or something else better than before - although I do appreciate your reluctance. No one likes to see their child in pain. I do have to say it takes weeks if not a few months to work out a good proper maintenace dosage of movicol because initially most people that start on it are in a bad way - pretty blocked up etc. That needs to be cleared properly and the bowel recover, before the bowel can start to retrain itself. Like I say my DD is 11 (just) and been in this now for 2 months - we are only just reaching a nice place with it now and I was the same when I started out.
Good luck.