I am not sure how double-clicking will prevent "... kids from buying apps". This is meant for ensuring the physical presence of a human and an intention while purchases are made. This reduces accidental purchases.
Probably you only have a Passcode. This is very much the same as Face ID and Double Click.
Settings > Face ID & Passcode > [Enter Passcode] > iTunes and App Store [Enable]
If you Enable Face ID on your device and enable access to purchases using your Face ID that will enable Double Clicking of the side button. That's it.