Always practice WITH your child when they first start to learn an instrument. Contrary to many people's beliefs, practicing does not come naturally, just like wanting broccoli over chocolate or cleaning their room over playing do not come naturally. Children need to be taught the virtues of commitment, focus and effort.
If you send your child away to practice in a room all alone, it's almost like punishing them. They will feel rejected and sent away and will not easily develop a love for the piano (or other instrument). But if you practice WITH them, then the instrument acquires an emotional importance, it's time spent WITH mommy or daddy.
After you have spent many hours practicing with your child, you can SLOWLY wean yourself away, first by alternating half a practice time with them, half without, then alternating whole practices by themselves and with you again, and so forth.
Remember, the fact that your child does not naturally gravitate to the piano does not mean they aren't interested or motivated. It's just that they are children and chocolate or playing is always more fun than broccoli of practicing.
Also, I would use the term playing piano with them and not the word practicing as playing piano sounds much more fun....