Setting Up a Secondary DNS zone in a mixed environment

I’m in a situation where we need to configure a child DNS so that only a zone (e.g., escuela.dominiox.yy) remains on a DNS provider other than Cloudflare.

After reading this article Set up a child domain - Subdomain setup · Cloudflare DNS docs it seems possible.
Am I correct?”

The article that you linked is referring to hosting a child zone on Cloudflare using an Enterprise account. You want to follow this guide that covers delegating a child zone to another DNS provider.