A beginner book will be good for you, not so much because books are necessarily the right idea for everyone, but because, generally, books have been written by an experienced teacher.. so there's reason in the progression of content. Following the book means you won't make any huge developmental errors. You need to observe that, be aware of what exactly is being taught as you move through the pages and make sure your student is learning that (as opposed to just what notes to play on that given page) - and why it should be in that order.
For example, a lot of books start with something to do with the black keys - this is because recognising the groups of black keys is imperative to the ability to locate/name specific white keys.
The bastien book has a tune, "two black cats" - so in learning it, as well as some basic rhythm the student will be able to find the groups of two keys.. a skill with is then subsequently used as the student learns the first white key song a few pages later..
Something you also need to take into consideration is that in order for you student to learn the song, they are going to have to practice it. With adults or older beginners, you can often say "lets repeat it" and that's enough because they understand and accept the objective is to better learn it. A 6 year old may think that its really boring to repeat something many times though, so its a good idea to figure out ways to encourage your student to repeat something without it becoming boring.
The other side of that is empowering the parents to be able to work with their child in a way that is fun and effective. The parents should be coming to lessons and learning how to practice with their child. You can't leave a 6 yr old by him/herself and expect effective practice.
....
One other thing with beginners, is to ensure that you have something for them to do during that first lesson that is actual music. A song. You can't spend the entire lesson trying to fill in basic knowledge.. they must be playing immediately. With youngsters (4-5 perhaps even younger though I don't take anyone younger than 4) "The chinese clock" is golden for that - its a duet, which means that they immediately have to learn to play with musical timing. It teaches the groups of 2 and 3 black keys thing, and its so easy a monkey could do it blind folded.
So you might take a short time to first discuss that the piano makes sound (and have a go at that), then learn that the sounds get higher to the right and lower to the left (which may entail teaching what right and left is), then that there are black keys and white keys.. and that the black keys are in groups.. which leads into using those groups to play "The Chinese Clock"
You'll find "the chinese clock" here (thanks to forum member "bernard") along with masses of other generalized info for teaching beginners.
https://www.pianostreet.com/smf/index.php?topic=2260.msg90216#msg90216^about that thread, don't raise your expectations too high expecting to achieve what bernhard says he does. He is a very experienced teacher (and gives everyday lessons, rather than once a week). The thing about teaching that you don't realise so much when you start is that
you have to learn about how to teach, not just what to teach. - the bonus is that it makes you a much better pianist yourself.