There are so many different things vying for our attention in our day to day lives that it can be extremely difficult to find time to practice. This goes for not only musical instruments but anything else that you need to do many times to get good at.
Here's the short and straight answer: If you have found something that you are truly truly passionate about, it's easy to forget about everything else, immerse yourself in it, and lose all track of time. You don't have to make time for your passions. If you're passionate about something, you will have to make time for other things.
On the other hand, if you are constantly at battle with yourself or your child about practicing, you're barking up the wrong tree. I'm not talking about a reminder every once in awhile, I'm talking about daily conversations, uncomfortable for all parties involved, about practicing for a half hour (the minimum I recommend).
Comfort vs. Discipline
Reminders are ok because anything that requires dedication and a high level of performance also requires discipline, and discipline isn't always flowers and candy. Sometimes you have to wake up earlier, skip that movie, or stay inside and put flesh to wood and put in some time on your instrument. But if you love it, what are you missing?
Here's the bottom line: anything is better than zero practice, but 1 hour is many times better than 5 minutes. Beginner students sometimes don't have the repertoire to practice any more than a half hour, but they need to learn muscle memory and posture and that can fill a half hour easily. If you're crazy about your craft, you can fill an hour easily too.
Here's my recommendation for beginners that are still figuring things out: Practice every day and take one day off during the week, but not the day before, on, or after your lesson. If you want to maintain your skill level and not improve, go for a half hour a day. If you want to improve, up that time to an hour or more.
Don't Scare Yourself
The reason I talk about such small numbers is because they are unintimidating and achievable. I see a lot of rookie players saying, 'I only have one day off so I'm going to do my whole week's practice on that day...6 hours!' That never works, and they just scare themselves into procrastinating or zero practice. Besides, it's almost detrimental to practice any more than 2 hours because you get fatigued and stop absorbing information. Take a 5 minute break every hour or two and do something different and active, like shoot a few hoops or walk around the block.
So next time you go to your lesson, be caught up on your work so you can use the time to learn new stuff, not to practice.