I had a promise ring at 14/15. It was colorful, so it didn’t look like any type of engagement ring. It was like $20 at Walmart. Shop off Amazon. Something colorful, or weirdly shaped, maybe has her favorite flower on it? You can find okay stuff from Amazon. Always look at reviews though, this way you can make sure it’s a good product. Also get something like size 6 - 7 and then ring snuggies, they’ll make it fit even if it doesn’t fit.
Maybe something like this: https://www.amazon.com/dp/B07FJRSQLZ/ref=cm_sw_r_cp_api_glt_fabc_K8XAZTC0NKAWFHYKHNA1