iOS uses a a 4-digit passcode to secure Screen Time settings (formerly known as Restrictions).
Use Screen Time Passcode
Use a passcode to secure Screen Time settings, and to allow for more time when limits expire.
I would instead like to use a 20+ character random password that is hard to remember and hard to type. I'll store the password and look it up when necessary, rather than easily accidentally remember the Screen Time passcode.
Is there any way to use an alphanumeric password rather than a 4-digit PIN?
This is similar to the main iOS device passcode, which does allow for more secure input:
Enter a six-digit passcode. Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code, or a custom alphanumeric code. (link)