The only exception I can see that some many of you are gritching about is something that is not even line CUTTING. If a child has to go to the bathroom and a parent has to take that child and leave for a few minutes, how tightly are you wound that you would refuse that child the chance to get back in line? Before you chant, "Make them go before they get in line," or "Tell them to hold it," here's a point you don't seem to be aware of. For some kids, that's just not a real possibility, medically speaking. When they have to go, they have to go. It's not a matter of training either.
If they were ALREADY in line, you lose NOTHING when they come back to the line in a few minutes. You haven't been pushed back in line and you're still the exact number of places behind the parent and child that you were before they went to the bathroom. Even if the kid went to the potty before they got in line, a kid with certain conditions can need to go again if he/she has been in line for 45 minutes. Have a bit of compassion.
I'm not one of those who think the world should bow down to people with children, or that a special set of rules apply to them. But common courtesy does not go out the window at WDW, does it? If you're on a bus and a pregnant woman or elderly person has to stand, an able-bodied person should offer them their seat. Common courtesy. If you realize there's a small child behind you who cannot see the parade, but will be able to if you just shift over a few inches, you move so that you can BOTH see the parade even though you had that spot staked out for an hour. Common courtesy. If a child has to leave the line for a few minutes to go to the bathroom and is only asking to get back into their same spot, which causes you zero disadvantage, I believe common courtesy should require you give that child a break.....and maybe even a little pixie dust....and just let him/her have THEIR place back.
I despise line cutters as much as the next person, but I would be ashamed of myself if I told a child that I wouldn't let them back in line because they had left to go potty. Line cutting is certainly rude, but THAT is far worse, IMHO.