My daughter had to wear a helmet too. My husband painted it pink with the Maisy character all over it (sorry, not Disney). There was a picture of a helmet at the therapist's office with Winnie the Pooh on it that was beautiful. The parents had an artist do it. The only problem is that the artist painted it with the helmet upside down, so Pooh was upside down on the kids head. Oh well! Tip number one: make sure when you paint the helmet you are painting it "right side up".
Tip number two: if I remember correctly, you will need to get the helmet on the kid relatively quickly after you receive the helmet. You might only have something like 24 hours to paint the helmet (and then will need to do any touch ups during that "one hour off" each day. Believe me, the "one hour off" goes very quickly because you need to use that time to wash your kids head/hair, scrub out the helmet with rubbing alcohol, dry the helmet, dry your kid's head/hair and kiss your kid's head all over

) so mailing it to your sister will not be an option.
Tip number three: I see that you are going to Disney during the warmer months. Keep in mind that the helmet will make your child's head very, very hot and sweaty. You are going to need to keep your son in the AC/shade as much as possible. My daughter wore her helmet all summer here in Wisconsin and we basically kept her inside during the hottest part of the day. We cranked the AC in the house to keep her comfortable at night. Some kids adapt pretty well and don't have the sweating issues. My kid did not... she would sweat like crazy!
You are really going to need to schedule that "one hour off" while in Disney because your son will need to have his head washed and helmet scrubbed out. You might want to consider bringing a small fan to use for drying the helmet out (due to increased humidity in FL) as it must be perfectly dry before you put it back on his head. You don't want him to develop any funky skin issues due to sweat/bacteria, etc... This can cause major problems with the helmet. My daughter developed a rash on her head at one point. We had to leave the helmet off until it cleared (to prevent skin breakdown). We were lucky that it happened towards the end of her treatment because by the time the rash cleared, the helmet didn't fit anymore. We just ended treatment a little early. Otherwise, we would have had to have a new helmet cast.
The helmet is a pain in the rear... it is a lot of work... but the results are well worth it.
This is probably way more information that you wanted, so if it helps, great, and if it rubs you the wrong way then just ignore my ramblings!
Good luck!