Actually all those things are extras. You don't need any of them to successfully teach piano to your child, however some you may want.
The one I will highly reccomend you get is the solfege stickers. You can use the printable piano key guides but I wouldn't attempt that age group without the stickers. They just need the extra support. So basically I am saying "buy the stickers!"I even recommend getting two sets as undoubtably at some point your son will pull some off ( don't let him see you sticking them on

) I suppose an alternative ( temporarily) would be to cut out and paste the symbols onto your keyboard.
The kingdom of tune book is a coloring in and story book to learn music theory. It is fun for kids, they do seem to like it. The decision to puchase it will be personal. How much do you know about music? If none, the book will be extra useful to you. You can always add it in later.
The flash cards you need right now are available free. Hopefully someone will post a link but I think it is under learning resources on the the home page. Te others you can download as you need them or want them.
Also there are lots of pretty free downloadable flash adds ( especially useful for Isaacs age group) in the share file section of this forum. Scroll right down to the bottom on the forum index page.
Oh and don't loose the crypto key! Tie a big ribbon to it. It is your program and expensive to loose!