As long as it's enjoyable, as long as it fits with the children's goals, as long as it fits with the priorities of the children and the family... I also don't think there is a fixed answer. My kids love music and are very advanced, at times I've definitely thought about pushing them to practice more. So far I've been able to suppress the urge. :-) They need time to develop in many areas. There are a lot of things that they should do in music as well, away from the instruments. In total, my DS14 probably spends 20 hours a week on music (lessons, pratice, work away from the instruments, ensembles, etc), and my DD9 probably 15 hours. When they were starting, though, at the age of 5 or 6, they were doing 15 to 20 minutes a day.
I've heard that some teachers think that the practice time each day should be the same as the length of the lesson. For example, if the kid takes a 45-minute lesson each week, then she/he should practice 45 minutes a day. But it depends so much on other factors as well.
Also, the quality of the practice is much more important than the time. I'd rather my kids have a focused practice for 20 minutes than a half-hearted one-hour session.
When the kids are young, I personally don't think it's a priority to practice a lot. I think developing good music sense, intuition, taste, listening skills, etc. are all important and lots of these are not done through practicing the instrument.
If you look at the now famous paper on deliberate practice and expertise by Ericksson (1993, a survey of violinists, the classic paper on "10,000 hour rule"), you will see that these violinists actually didn't practice much in the first few years of study (on average, probably 20-30 minutes a day. They picked up the pace dramatically in teenage years. The vast majority of the kids won't be professional musicians anyways.
Ensemble: my kids really enjoy it since it's fun for them to interact with other musicians and they get to work on a wider range of music. They started at 8 and 6, respectively.
