From the time my children were little, they got
25¢ a day for their practicing.
That positive reward is much better than scolding them for not practicing and making music a negative experience
for them and for you.
For the first few months, they will still needed daily reminders.
For years they will still need occasional reminders.
THIS IS NORMAL
Over time they will begin to develop self-discipline.
No one is born with self-discipline, it takes years to develop.
Find the reward system that works for you.
Maybe a week of daily practice is a trip to the ice cream shop.
Do whatever works for your family and is also positive.