Nappys

I'm not sure where you buy them if not a pharmacy but there are like tenna pads that children with complex needs use that I work with use they start at size xs and go up ill see if I can find them for you x

There will definitely be some that exist as my friend’s brother was in nappies at 11 (he has ASD). I can’t help any more than that, though 😭

Pampers go big on sizes

Nappy pants have bigger weight limits so a size 8 nappy might be the equivalent of a size 6 nappy pant

I second abena, my son has these through the incontinence team, they're very absorbent. You could also go down the route of incontinence (can't remember what age they need to be) they'll provide them for free for your little one if she's struggling with toilet training. I had occupational therapy refer him to them. May be worth an ask.

Have you tried Fred & Flo size 8 nappy pants? They come up quite big, my 6 year old wears them. Tesco also do their own brand of larger nappies for older children but I've never tried them

You can get them on prescription, ask for a referral to the incontinence team