Great question/topic, and if the definition of password "management" here also includes "password generation" I would provide a Python program or similar that generates cryptographically-secure passwords of various lengths with just a few lines of code, and show the child how to easily run the program anytime they need a strong password. (as this would be a better habit in my opinion than thinking of a strong password each time).
Here is a rudimentary example I built that is cryptographically-secure using the secrets module in Python and library of 64 characters: https://github.com/hatgit/hatnotation/blob/master/Hatnotation-Password-Generator.py
Such 'secure' passwords, however, cannot be easily remembered as they look like a string of machine-readable code, unless the underlying binary is converted to mnemonic words so it can be easily written down.
I've also built an encoder/decoder (notation system called Hatnotation) and with the above compatible password generator for educational purpose, the underlying binary could be pasted into a mnemonic code converter, such as follows:
A random 22-character password generated from the range of 64 characters where 64^22 == 2^132 in terms of bits of security/entropy, using the Hatnotation password generator:
Underlying 132-bit binary that represents those 22-characters (non-ascii, these are Hatnotation- encoded characters:)
Converted 132-bits into a mnemonic based on the BIP39 English wordlist (excluding checksum, and which can be an alternative to the Diceware options that @Forest provided), using a mnemonic converter that can work offline on a standalone basis:
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An alternative is to teach them to how to generate entropy in binary or hex format using the command line or code compiler (which is faster than flipping coins), and how to paste such binary into the mnemonic converter of their choice depending on the wordlist used (even if it is their own custom wordlist). In Python, there are a few secure ways to do so using the built-in
P.S. In terms of storage/password-custody retrieval, if passwords are saved in the browser's native manager, and there is a concern about logging into the browser (i.e. Google Chrome) session from public or other potentially-unsafe locations in order to access passwords for logging-in to services such as the Scratch.Mit.edu website , I think adding a 2FA app such as Google Authenticator (assuming the youngster has a cell phone or compatible device, even if it is offline) could reduce the risks of those passwords being accessed by an adversary, if the login info was otherwise compromised.