While the perfect time is different for everyone, here are some suggestions to get you thinking:
1. In the morning. While many of you are probably thinking "no way!", this has always been my best practising time. When I was growing up, I'd practise while my Mom was braiding my sister's hair and vice versa. Of course as I got older and more advanced, this became a longer session, but these initial short am practises were what created the habit.
If you practise in the morning, there are fewer distractions and reasons that you might NOT practise!
So try it! Set that alarm 15 minutes earlier and have a piano pyjama party!
2. Before dinner. This was another good one for me growing up. While my Mom cooked (and listened!) from the kitchen, I'd run through my pieces. She's shout out compliments a few times as I went along. For kids who are past the beginner stage and wanting to practise on their own, this can give them a sense of independence, yet enough involvement (ie positive feedback) to keep it exciting. Plus, the routine of piano, then dinner creates a good habit.
3. Before bed. This is not a very effective time for me, unless I'm composing or doing something creative, but it can be a very relaxing way to end your day! For kids practising at this time, you could always let them "stay up late" to practise just a little bit more!
I hope this gets you thinking!
What routine works best for you? I'd love to hear your comments below.
For reading on some very interesting (and often extreme) habits, check out the book Daily Rituals: How Artists Work by Mason Currey. This was one of my favourite summer reads.