I don't need to impose parental controls on my own phone, so I'm not extremely familiar with them. I am throwing out the following as food for thought.
If you are worried about your child's photos "staying only on her cell", disabling iCloud Photos is probably just one of your worries. The basic iCloud Photos is for syncing photos between devices using the same Apple ID – so she would need to give someone else her Apple ID and password for them to get at iCloud Photos from another device. If she's giving out her Apple ID and password, there are other things you need to worry about …
What if she
- Sent photos in iMessages or SMS/MMS messages?
- Sent photos as attachments to e-mail?
- Uploaded photos to social media sites, either using mobile Safari, or using dedicated apps for social media sites such as Facebook/Meta and X/Twitter?
- Contributed photos to an iCloud Shared Photo Library?
- and so on, and so forth?
Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support
I think there may be parental controls that would let you restrict some of the features on her phone; e.g., specifically denying access to her photos to most of her apps, on an app-by-app basis – and locking down restriction changes, so that requires a passcode that you know, and she doesn't.
Also: Be aware that iPhones normally attach information about where a picture was taken, to the picture. This can be a great convenience when organizing your own photo library, but you might not always want others to have that information. The Export options in the Mac version of Photos have a checkbox that you can use to determine if you want pictures exported from your Photos Library to contain this hidden information.