I know from experience that if you have to exit a line with a preschooler who needs to go to the bathroom, a CM WILL let you back in line, and they WILL let family members hold a place for you, etc. They'd much rather do that than have the kid "let it go" right there. You'll get the stink eye from other guests on the way out and the way back in, but that's life if you go to WDW with a 3 year old. I've had to run out of line at least once with every 3 year old I've brought to WDW, even right before boarding, even with my littlest who had been very reliably potty trained for a year at that point.
I will say this - one night a few years ago we were waiting for the Disney bus, it was probably 10 at night (it was already very dark), and a little boy about age 6 was just dying from having to pee so bad, hopping from side to side, etc. I knew it was minimum 30 minutes before the boy would get to a bathroom, factoring in the bus arrival and ride. None of us knew where a bathroom was at the TTC, it was too dark to see, all we could see were the bus stops and bushes for what seemed like at least 50 yards, and there was no CM to ask. His parents kept telling him to tough it out. I leaned over and told his parents that I'd taught elementary school for 7 years, and in my professional opinion that boy wasn't going to make it another three minutes, let alone a half hour. Told them if it were me I'd send him to the bushes over in the dark or there would be a puddle on the bus. He went, I don't think anyone else even knew. Sometimes you gotta do what you gotta do.