Hi OP,
Please take a look - even a brief look - at this science ( especially the graph on page 5):
https://sapienlabs.org/wp-content/uploads/2023/05/Sapien-Labs-Age-of-First-Smartphone-and-Mental-Wellbeing-Outcomes.pdf
I would put off getting your child a phone for as long as possible.
If they really need it, buy them a flip-phone.