20 hours is far too much for 6-8 year olds. If the goal is to get these kids to elite, they need to still be in the sport in 10 years, where they are old enough for senior competitions at 16-18.
Excessive training hours on very immature bodies will pay the price later on. When they are young you probably won’t notice the physical issues but the stresses in their bodies will be occurring, and then when they hit puberty the problems related to a lot of stress on their soft bones over the years will start to show, and you will usually Get chronic injuries.
Not to mention burn out, not just the kids but the parents and families. At 6, training 20 hours a week and doing school is a lot, so many parents will opt to home school. 10 years of that will take its toll on the parents too. Not that home schooling isn’t loved by many families, and they make it work well for their kids. But if parents feel forced into by the sport, then they often expect the sport to pay them back in many ways and they become disillusioned.
I would keep, it at no more than 10 hours a week at 6, and stepping up a little at 7 or 8.
Group size obviously plays a role. A large group without multiple coaches will make it hard but you don’t want the group to be too small either. If the group os too small it can easily lose its emergy and dynamic. The kids feed off each other and learn from each other.