The phone needs to be unlocked, GSM, manufactured for use in the US, and certified for VoLTE by T-Mobile. (Gee, sounds simple until I typed it....)

The safest new phone would be one that is factory unlocked: This Motorola Moto E is fairly inexpensive at $120, has all the important bands for T-Mobile, and can be used on any other US network should your needs change.

https://www.amazon.com/Moto-Unlocked-Motorola-Camera-XT2052-1/dp/B086H3HH5V

Potential drawback: it may be a little large for a pre-teen. Look at the dimensions.

Which phone did you get? I saw it on Slickdeals but then saw so many people getting SIM error and having to email customer service to get the rebate so refrained myself to post about it here lol.

I am thinking of buying this Moto E - https://www.amazon.com/Moto-Unlocked-Motorola-Camera-XT2052-1/dp/B086H3HH5V

Or G7 Play - https://www.amazon.com/Moto-G7-Play-Unlocked-Warranty/dp/B07N9MF1Q3