As with teaching children anything, give them more if they show interest/motivation and let them do other things if they don't care much. Chess is especially time consuming in study, games, books and so on. Teaching the moves is a great start 
If the motivation in them is still there, then I'd probably introduce basic checkmates and basic theoretical endgames. Both of those are enough "endgame study." For "opening study" I wouldn't even teach openings beyond maybe the first 3 or 4 opening moves. "Opening Principles" is enough. As for "middlegame" probably just tactics is enough for now.
All of what I just named could be enough information for a few months at least. Take "theoretical endgames" for example (if they are motivated to learn chess), try teaching them to win with Q + K vs K, then maybe R + K vs K. I even like teaching children K + pawn vs King.
We'll see if you both get that far, but most of the time the children motivation shifts to something else (not chess) and that is okay too. A lot of the time they are either just curious (in this case to see what chess is) or they just want to spend some time with someone (in this case playing chess).
Good luck with your chess teaching
It's Christmas and my uncle asked me to teach his 9 year old daughter chess. I agreed, i showed her basic piece movement, we played a few games and she got it pretty well but I have no idea what to do next. We will play more games for sure but i taught her just movement, no castles, no en passant, no promotion, anybody knows when to start introducing new stuff? And how to do it? When to introduce tactics and openings? She's not really brilliant so I don't want to push.