A 7 years old has normally tiny hands (and usually also small pockets), a "full size" phone might be not much handy, if it is an emergency only phone you can get any of the "feature phones" (not iOS, nor Android) that just make calls and sms.

There are el-cheapo Nokia models, as an example.

If you are going the smartphone way, there are just a handful of "right" (small) sized phones, Unihertz and Cubot come to mind.