To note, the “easily accessible” part is for people with an Apple device with iCloud set as the target account.
You don’t have another Apple device to manage your kid’s subscriptions ? it’s simple ! Download iTunes on Windows! No windows or can’t/don’t want to install iTunes? Tough luck.
You’d think icloud.com has feature parity regarding payment managements and critical settings, and no, it doesn’t [0]
BTW having multiple Apple devices still won’t make it that much easier if you happen to use different Apple IDs (if you need access to more than one country’s store for instance, or separate work and personal profiles): logging in and out is a PITA with the Find My Devices lock and 2FA.