At no point did anyone state it’s a device belonging to someone else.
a father can let their child use their iPad temporarily. Does it make it the child’s iPad now?
You did state in the original post that it was the child's iPad, not the father's.
Anytime anyone has access to someone else's device, they're free to delete things they ought not delete. However, that's not the point of what happened here.
Two people, each of whom should have separate data/separate data storage, who shared a single Apple ID/iCloud account. Anytime anyone shares a resource there's potential for issues. If this was a refrigerator... The kids come home from school, grab some "snacks," and end up eating something Dad or Mom was planning to serve for dinner.
iCloud is designed to synchronize data among multiple devices - the same info on my iPhone, iPad, and Mac. If I update that data (whether add or delete) on one device, it's intended to update on the other devices as well.
This works great for one person, but when people share a single iCloud account it means the same data will be available on every person's device(s) and can be modified by any of those persons. The "my kid deleted all my contacts" is an all-too-common story.
The answer is a separate Apple ID for each individual. Family Sharing allows sharing of apps, music, subscriptions, and even a large iCloud storage plan (where each person's data IS kept separate) among the family members. Family Sharing also includes features that allow parents to remotely control their children's ScreenTime/restrictions and approve/decline their children's App Store and iTunes purchases.
