Most injuries on planes occur on the ground. They do, in fact, jerk and move unpredictably on occasions. Furthermore, the pilot and flight attendants absolutely need to minimize distractions at this time. One distraction may not seem like a big deal, but what about the other 20 cranky people who want something? Suddenly the flight attendants are playing zookeeper rather than focusing on what needs to happen to keep everyone safe.
I can sympathize that you can’t always time the bathroom right with a kid. But pull ups exist for a reason. I’m not blaming mom here, as it seems like a simple miscalculation. But it’s also not the airlines fault.
Personally, I think the pilot made the right choice. If someone doesn’t follow directions from the flight crew, they need to get off the plane. Clearly, they stabilized the situation before takeoff without having to unload the unruly passenger. Maybe they didn’t handle the situation ideally, but the mother is a moron.
The mother is a moron because she thinks it’s more important to clean up a little pee than to follow instructions from a trained crew. Pee happens. It’s not fatal. If, while unbuckled, something had happened and she’d broken her neck and been rendered a quadriplegic, she’d probably sue the airlines.
That said, it’s possible that the pilot and crew might have taken a better option: ask the mother to wait a minute (before the peeing), radio to tower that they’ll be delayed 10 minutes, turn put on the brakes (assuming it’s a place where that’s legal), allowed the mother to take her child to the toilet, and then radioed back “ready” when she’s back in her seat. Of course, this also delays all flights queued up behind.