Some students have a natural sense of rhythm, while others are a bit deficient in this area. I firmly believe that, just like learning to sing on pitch (I don't believe in tone deafness), rhythm is a skill that can be learned. Like many skills, it is more easily acquired at an early age, but it's never too late to work on improving in this area.
I wholeheartedly agree with the earlier post that rudimentary rhythmic work can, and even should, be done away from the keyboard. When I was teaching group classes, weeks before we introduced a new rhythm in a new piece (such as dotted quarter-eighth), we would sing songs with it, play it on rhythm instruments, dance to it... By the time we got to the piece that used the rhythm, they would have the sound of it firmly in their ears. All that would be left would be showing them the notation that represents it.
It's true that, one day, we want them to be fluent enough in notation that they can start with the notation and figure out rhythms that they may have never heard before. We shouldn't always be spoon-feeding them the sound first and then showing the notation. But in the early stages, I think it is more helpful to start with the ears and then go on to the eyes.
And by all means, try to engage the student in as many rhythmic activities as possible that don't involve playing. In this case, since we are talking about your own child, you can do things like tapping the beat whenever you are both listening to music together. Or practice moving around the room, taking steps on each beat while reciting a rhyme or singing a song. Some students very much link movement to learning, and the key to their rhythmic understanding is provided through movement activities.
Be patient, but be persistent. Progress will come in its time.