9 hours a week sounds a huge amount, for any recreational/sports activity especially when you're not even 7 yrs old.
Personally, I wouldn't do it for my DC, but I know with gymnastics the 'peak' is often in the early teens so huge sacrifices must be made in the pre-teen years...
I know this because I did competitive gymnastics from around the age of 7. In the end it got too intense and taking up disproportionate amount of time
I stopped aged 11 and then pursued other team and individual sports which I really enjoys and excelled at
Also, during the gymnastics years I developed what I now recognise to be an eating disorder- starving myself, getting up early to exercise, lying to my parents about having eaten at school, hiding food & throwing it away.
I was obsessed with my weight and losing more...I remember this so clearly during my last year at primary school...
Thank god I stopped these behaviours after giving up gymnastics and ate normally ever since, but looking back it was scary how I was showing all the signs of anorexia
This is just my experience, but for my DC, we're encouraging them to do other sports instead