I'll get flamed for this one, but my oldest is now in his mid-20s, and we have an international family, so we traveled a LOT with kids. Because I knew that delays happen all the time, I made it a point NOT to allow my kids to get hung up on a bedtime routine when they were little; we had strollers and slings and just kept marching, and they fell asleep when they were tired, wherever they were, and regardless of what was going on. Disney was no exception. (They did tend to try to stay awake whilst still in the parks, but the minute we were out the gate their eyes would fall shut in the stroller. After a while we learned to change them into PJs and brush their teeth in the parks before the fireworks, so that we didn't have to try to wake them again before properly putting them to bed in the hotel room.) Our usual WDW routine when they were little was to leave the park in the heat of the day for a swim so they would fall asleep afterward, and then we would go back to the parks at about 5 and stay until closing, with them sleeping in the stroller for the final 2-3 hours of that. Sometimes we had to briefly exit the park to get them to let sleep come, but after a 15 minute walk in the parking lot we could go back in.)
IME, total exhaustion is the key, and as long as you run them off their legs, most little kids will fall asleep standing up if that's the only option. My eldest was (and is) legendary for his ability to fall asleep anywhere if he is tired. He has several times fallen asleep in the middle of up-close fireworks shows.