I'm a high school history teacher here in Canada, and have also completed a Master's degree in history, so Ill pass on a couple pieces of advice from those experiences. The very first thing I would recommend doing would be to get your hands on the curriculum documents for their appropriate grades if they were in a public school system and go through the expectations for history (or social science as it might be listed, depending on where you live). While you might wish to expand on certain area's/ customize for their particular interests in a home-school setting, it’s important that they receive a grounding in the same areas that their peers in a public school will receive.

That said, I think it’s great that you are going to begin with a "what is history" style section. It’s very important to cover this kind of material with a history class, and I like to spend some time on it at the start of all my history classes, either extensively or as a refresher. With this age, Id focus on some of the very basics, such as the difference between primary and secondary sources, primary being first-hand accounts or evidence and secondary being what has been written by historians later on based on both primary sources and other secondary sources. I would also focus on the notion of bias, or how historians can read and use their sources and present them in such a way as to fit their own narrative. Later on, you may want to come back to this and introduce the major historical trends and perspectives that have existed in historiography (the writing of history), such as the Whig interpretation of inevitable progress or the Marxist interpretation of history as based in class conflict. But these are lessons for a later day (or year even). At this age (10-12) I would say an understanding of the difference between primary and secondary and the idea of bias is sufficient to start getting into the more engaging (for students) events, and people of whichever area(s) periods you’re looking to cover.

In terms of teaching resources, Ill recommend you to the education ministry where ever you’re from. There are often databases online where teachers share lesson plans and resources available there. I quite often use a UK site, https://www.tes.co.uk/teaching-resources/ to help come up with idea for lesson plans and exercises on the topics I cover in class. You may not find exactly the lesson you want to teach there (in fact I’m sure you won’t, because it always important to gear lessons to your specific group of students) but you’ll more than likely find something inspirational or that you can adapt for your students.

Finally, for your own background, two university level books that Ill recommend are E.H. Carr, What Is History? (1961), and notable response book Geoffrey Elton's The Practice of History (1967). These are books that are waaaaaayyyyyyyyy above the level that you will be teaching at, but they are probably among the best reads to satisfy your own curiosity about the discipline of history and how historians know what they know (if in fact we do haha). Richard Evans’ In Defence of History (1999) is another excellent read on the same subject, if the first two books happen to pique your interest.

I hope all of this is in some way helpful to you. Best of luck!