Not a teacher, but keep in mind reward-based exercises (like treats and candy!) and making piano fun for them (it already is, though) will help motivate them to work harder and view practicing as fun time.
Few interesting comments in here..This isn't going to be that applicable because you've already started, but with a child this age the first thing I do is just identify that there are groups of 2 and 3 black notes and then we IMMEDIATELY play a tune (teacher/student duet) that uses both hands for the black notes only.This tune (stolen from bernhard) sounds like its title. So you can immediately start encouraging use of sound image, it also gets the child focusing on a sense of pulse/timing because they have to play together with the teacher. I usually just improv the teacher part which gives me the opportunity to test how well the child can keep/identify where the beat is amongst increasingly complex accompaniment. Also, where possible, I teach the PARENT how to play a simple version of the teacher part so they can do it at home.Once we've done this we can start talking about identifying white notes by name (maybe just 2 notes on day one, C+F except where there is a specific difficulty and they do better with just D) I play "whats that note" which basically just means I play a note and they tell me what it is (and vice versa, where on occasion I will give a wrong answer to see if they pick me up) - and most importantly I present it like that "Now we're going to play a game! its called "what's that note?" ..these are the rules. I also use this as an opportunity to explore the pitch extremes of the keyboard which means they start identifying octaves and realising that there is high and low pitch and that they work left/low right/high. At the same time I also start saying "can you play the next note loud (or soft).Then we might go back and explore "the chinese clock" in different octaves, and with different dynamics...and then we might just free improvise, but I will give them some direction such as white notes, and with a certain simple rhythm so that I can play some chord progression underneath that will sound decent and in time. This will usually be quite a mess at this stage, and thats fine.. I'm looking for "noise exploration" not refined music... ..also, parental involvement is COMPULSORY at this age, I have usually told parents that before they make it to the lesson and that if they are not able to be involved then they should expect extremely limited progress and either find a different teacher or come every single day.