@ScaredOncologyMum If anyone on here has experience of that particular treatment I'm sure they'll let you know. I know it's used for colon cancer and I see it's also used for a variety of other solid tumours. I do have colon cancer but I'm on FOLFOX rather than FOLFIRI so no experience except for being told it can further worsen diarrhea which is already an issue for me with an ileostomy, so they'd rather have me on Oxaliplatin despite it being a nasty little bitch of a drug.
I don't know whether you've had any luck in the Children's cancer threads or the Life Limiting Illness threads, if not you may have to seek beyond Mumsnet. There's also a long-running thread called the Storm, in the life limiting illness section, though they state it's mostly for partners.
As you're aware we are a support thread for patients with incurable cancer so this is not the best place to seek support if you are not the patient. Many on here are mums and will empathise with your situation, but we are not set up to provide support for family members. Also, there aren't a lot of regular posters and so there may not be anyone on here with specific helpful experience.
Does googling the drug name with 'forum' or some such bring any helpful results at all? I just did and got a lot of results, not necessarily exactly relevant to your situation but you may find something useful, here are the first few, apologies if you've seen it all before:
https://cholangiocarcinoma.org/db/topic/experiences-with-irinotecan-folfiri/
https://community.bowelcanceruk.org.uk/topic/7582-irinotecan/
https://community.bowelcanceruk.org.uk/topic/642-irinotecan-and-5fu-your-experiences/
https://community.macmillan.org.uk/cancer_types/bowel-colon-rectum-cancer-forum/f/diagnosis/231222/irinotecan-chemo-drug
Always bear in mind that not every patient will get every side effect so while research is helpful and it's always good to hear experiences, it's not necessarily a good predictor of what your daughter will experience.
In real life are there palliative doctors or nurses available to talk to you? They might have helpful information and advice and be able to talk you through the risks and benefits, even if it's not the patient experiences you want to find.
Also, what do the oncologists say? If they feel it is worth treating her it's likely because they have hope that she can have increased length and quality of life so that's something to bear in mind. In my experience of oncologists, if they felt they couldn't be of any further help to her, they would say so.
Best of luck with your search.