Love this. A few things from Signal's own docs that help with a kid's phone:
1. Signal's Terms set the minimum age at 13, so it's worth her mom being the one who sets it up and keeps an eye on it.
2. It still registers with a phone number. In Settings > Privacy > Phone Number, "Who can see my number" defaults to Nobody now. You can also set "Who can find me by my number" to Nobody, and then people can only reach her through a username she shares (it needs two digits at the end).
3. Turn on Registration Lock (a PIN) so nobody can re-register her number on another phone.
4. Disappearing messages and blocking are per chat if mom wants them.
Sources: https://signal.org/legal/ and https://support.signal.org/hc/en-us/articles/6712070553754
