Karen -

One thought is to just wear something sleeveless as you go through security. If that's not something you're comfortable wearing on the flight, you can always change in the first restroom on the other side of security.

Also bring multiple copies of the doctor's note: for as many check points as you will have to pass through on your journey and back, where screeners may want to hold onto the letter.

I don't know if Canada is using "advanced imagery" (x-ray images down to the skin) but if she is, be aware that bulky clothing is more likely to trigger an alarm. So if you can tolerate something "form-fitting" even for just the few minutes it takes to go through security, you are better off than not.

And at least in the U.S., passengers with special medical needs are entitled to pat-downs in private areas, with the passenger's option of removing some clothing so the pat-down is done "lightly" through a paper smock of some sort. But if someone may have go that route, s/he should allow extra time for getting through security, possibly as much as another hour. I know, not fun with a two-year-old. And speaking of which, the pat-down rules apply to little kids as well, so best they be dressed accordingly and told in advance what to expect, and that it's okay [which it isn't] and you'll be there the whole time, etc.

Finally, I would go on a government website to see what the current limits are in terms of fluid ounces (or ccm's) of anything liquid you can bring on board. No sense having to toss the juice boxes!

Mike