Answers
How do I know if a messaging app is safe for my child?
Check three things: whether a stranger can find them, whether a stranger can message them, and whether harmful messages are stopped before delivery or removed after a report. The marketing will not tell you; the settings will.
Most parental attention goes to content filtering, and most harm arrives through contact. A platform where a stranger structurally cannot find or message your child removes the largest category before moderation is involved at all.
The second question is what happens to what does arrive. Removal after a report is bookkeeping — the message was read. Screening before delivery is the only version that prevents the harm.
The third thing to check is whether your child can block somebody without involving you. One who cannot, will not.