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. The plugin settings
Site image will be one of the heaviest assets on your child site. The easiest way to speed up a site is to have a CDN compress images when uploaded, and then have the CDN serve those site images for users of the site. One solid Image CDN which has an easy-to-use WordPress plugin is Gumlet. First, you will need to sign up for a Gumlet account. After you have signed and verified your account, you will need to setup your web folder source. The source for the web folder will be the site URL, without the trailing/at the end
Cloudflare is among the most solid CDNs out there and has a considerable amount of data centers. To utilize Cloudflare for your child site, you first need to change the domain’s DNS name servers to use the Cloudflare name servers. Now that your child site is using the Cloudflare name servers (which you can check from DNS Checker), the next step is to set DNS and enter the site domain. Cloudflare will show orange arrows for the domain in DNS, which means Cloudflare will be used. Cloudflare also has an option to check that the name servers are being used.