If you are looking for a free solid CDN option for your child sites then one of the best options out there is Statically. Statically is powered by Fastly, Cloudflare, BunnyCDN, CDN77 which means it has a higher number of PoPs that will be serving the site assets.
Statically now has a plugin for WordPress which is very easy to setup. After you have installed and activated the Statically plugin you will need to go ahead and add your email address, site URL and then you will be able to generate an API key for the plugin.
World of WordPress is hosted by:
While others try to add more components to their hosting to make it go faster, we decided to build our hosting cloud fast from the ground up. We did so by stripping out everything we knew was deadweight.