My oldest lad started when he was 11, but he was tall enough to wear small/medium kit, but I had the problem finding kit for my youngest lad ,he was much shorter than his brother when he started at 11.
Even thou companies do youth size, it was the empire kit that fit him without him looking like a walking tent lol , jersey, pants and bounce vest.
The setup i let him use was an Invert mini, 0.8 bottle , and at that time , used a pulse loader, which looked huge when he held the setup. Got him some grills which most important was being tight enough to fit round his head, and a 3.2.2 NXE pack which fit ok round his waist.
I would'nt worry too much about baggyness, what is most important is the mask being secure. At there age, they will grow quickly, so if you buy some kit that fits good, in 6 months time they may need bigger kit.
heres a vid of my youngest with kit when he was 11