I totally agree with Rainbow324- some children thrive on being busy and can cope perfectly fine with the late nights, whilst others struggle.
My DD (just turned 9) moved from a Development squad doing 4 hours a week to a higher squad training 10.5 hours a week when she was 7. When she was 8 she was moved into the top squad at the gym and now trains 14.5 hours a week, including 3 nights finishing at 830pm. The only struggle is starting at 8am on Saturdays after finishing at 830pm on Fridays. She absolutely loves gym - whenever the gym run badge camps in the holidays she begs to go.
Contrary to a PPs concerns, it hasn't stopped her being "rounded" at all and she does loads of other activities. She is also in a competitive swim squad and has passed stage 10 swimming, does 4 different dance styles, is learning an instrument and is in the school choir. She's working at greater depth across the board at school and has a reading age well above her actual age. She also has many friends and goes to parties and on playdates.
Also as Rainbow said, there are so many benefits. My DD is so fit and has abs of steel 😆. She is disciplined, motivated, takes instruction very well, is very confident, and has friends of all ages.
It really depends on your DD. Does she love it enough to put in those hours? Will she cope with a late bedtime? Does she like being busy or does she prefer having lots of downtime?
Good luck with your decision.