It's kinda sweet for you to pack clothes for the girl but there's no incentive for the child's mum to do it if she knows you will. I've had to call parents to bring spare clothes to a kinder session because their child is wet or muddy and I've run out of spare clothes that we keep at the kinder. I've also had to put children in dress ups so they at least have something on.
It's not your responsibility though.