I've made plenty of mistakes as a parent. We've all done..
What I never did was taking my eyes from my kids when they were toddlers except on the care of my DH who was even more of a hawk than I was.
A disabled 3 y.o with disabilies should be constanty monitored for safety in public outings. A mistake in watching a phone, having a chat with someone, or doing something, whilst losing track where he is can mean serious consequences.