Write down everything that has happened to her physically. Write down her diet. Explain her fluid intake and how this is low because of the impaction, not before the impaction occurs, but as a result of impaction making it hard for her to take anything in. Writing it down and having the checklist with you and a pen to tick off everything you say will make you clear and concise and help you relay everything that is important.
Tell the team that she is seeing that you want an examination to see if her bowel is working correctly, not just treatment for impaction.
It's clear she's impacted. She is distended and has overflow (the leaking you are dealing with). You already know this.
What you want to know is
a. why is this happening
b. how can you prevent it happening again (based on knowing why it is happening)
Be clear. Yes you are emotional right now, especially as you feel unheard and your child is in distress. But do not muddy the waters with your worries. Just stick to the facts.
I would still want to take her to A & E if yet again no one wants to investigate why this is happening. The OOH doc said she needed an xray. If you don't get this from the team you're seeing today, then go to A & E anyway. Get your dp to take her if you think it will be better for you to stay with your other dc. Unfortunately you will have to wait. But take some toys and a blanket and buggy to keep her occupied. Stick to your guns.
It's really ok to get a second opinion. Doctors who go to doctors do it. Complaints should follow a formal proceedure too, not a callback from the gp you complained about telling you to keep quiet. That cannot be part of the official proceedure.