See your GP again and ask for a referral to paediatric Gastroenterologist, my DS had long term chronic constipation with overflow diarrhoea, it is very common in children, until the impaction is released it won’t improve and your DD will be in pain and completely unable to control the overflow diarrhoea.
My son saw two different consultants both privately, the first one didn’t help we tried again and it is now completely under control.
It was such a worrying time for me, so I understand what you are going through.
My DS impaction routine was 6 sachets of movicol paediatric daily for 4 days each morning, and 20-30mls of sodium pico sulphate daily, messy but cleared the blockage, obviously he couldn’t go to school!
Daily now he has 2 sachets of movicol paediatric each morning and 1 spoon 5ml of sodium pico sulphate.
He is absolutely fine now. Not further problems as long as he keeps up the medication he is regular.
Good luck OP, if you can afford to see a paediatric Gastroenterologist privately for quickness it’s worth it. Good luck I hope it’s sorted out soon.