[codingdave]

For a 7 year old, I would not start with coding. Start by showing her the dev tools in the browser. She'll have a blast changing the text, colors, and layouts of web sites. If she is like my kids were at that age, she will laugh at everything she can change, while also gaining the knowledge you said she enjoys - understanding how things work.

If she is still interested once she understands what a web site really is made of, then teach some basic coding. There are plenty of resources for that, code.org being a decent one that has already broken it down into age-specific lessons.