My ex shows up to my sons class every morning when I drop him off. It irritates the hell out of me and I would love to tell her to stop. But I can't so I just go along with it.
I understand your point-if something was going to affect the health or safety, you might step in. Some one mentioned a car seat not being present. Well, what if the child is on the border of needing/not needing? Mom says yes, dad say no? What is mom going to do about it? It's a parenting decision that one may not like.
This whole debate has kind of got off track. My point...if dad is gone, and the dad only see's the child every 3rd weekend, and the step mom is a positive influence, as well as the siblings, then I think the child should go visit. Some may disagree by pointing out the law-but you also have to look at the spirit of the law. My order has written "any competent addult we deem necessary for pick up". Maybe others do not have that.