My girls haven’t needed braces yet and are really tall (at least they got one good thing from my genetics since I have eds also). I found these on amazon and not sure how much it’ll help but might at least add some support
Bucwild Sports Youth Compression Arm Sleeves (Pair) Football Baseball Basketball Cycling Tennis Royal Blue Camo Youth Medium YM https://www.amazon.com/dp/B075Q3VKVL/ref=cm_sw_r_cp_api_i_EjqRDbR2T1452