My little one also has Sever's and wears the Tuli's Cheetahs during flare ups. We wen't with the sized versions for the same reason sportzmomov3 mentioned. I help dd put them on before gym and she wears them with flip-flops into gym, and then once home I help her take them off. I pull them off inside out, and leave them inside out over night to air out and keep from getting stinky.
It also has become part of our "sever's ritual." When she has a flare up immediately following the removal of the Tuli's dd does her stretches for them, showers, and then takes an ice pack to bed where she does the ice/elevate as she goes to sleep. In the morning, seeing the inside out Tuli's there reminds her to do her stretches again while I prepare her breakfast.
BTW- I only let her wear the flip flops with the Tuli's on. No more flat soled shoes for her unless she has been flare-up free for over a week. She is a Tom's and Ugg's girl, but she has to choose between the sport and fashion. If she is not willing to do her part, I will not be writing any more checks.