There are two ways, depending on whose Apple ID the child's device is signed into.
The device has its own Apple ID (your child's own account): use Family Sharing. Go to Settings > Family Sharing, tap your child, choose View only, and enter the email or phone number of your child's iCloud account — a QR code appears. Your child scans it with their Camera app, taps "This is [name]'s device", and their device locks to their own view — they can see their money and history, but iCloud itself stops them changing anything.
The device shares your Apple ID (a family iPad, or a hand-me-down phone on your account): the data is already synced there, so use an invite code instead. Go to Settings > Child Codes > [Child's name] > Generate invite code and enter it on their device.
Both paths end in the same kid view. Setting up a child's device is a Premium feature either way.