I don't think you would be legally responsible- but if the other kids' parents weren't around, I would definitely speak up! And- if you know who they are, I would let their moms know about the horseplay.
Not in an accusing way- but just 'I wouldn't have said anything, but they were in the road and just really not paying attention. I was the only adult around and I wasn't sure what to do, but I thought you would want to know." Most moms can't take exception to that, as long as you aren't criticizing them.
In the bigger picture- you CAN'T be the only mom frustrated by this. I would get together with other moms at the school and contact the DISTRICT. If you take it to that level, they will take you more seriously. If you are in a group, they will take you more seriously. Ask specifically for the issue of :
1)Lack of crossing guard
and
2)Why walkers must wait outside alone unattended, while bussed kids are allowed indoors earlier?
to be put onto the agenda of the next board meeting. Then ALL of you show up and get as many people to speak as possible. Your tax dollars are paying for the school- you ought to get a say in how they are spent!
if they can't afford crossing guards, that is one thing. But it is absolutely unacceptable for one group of students to be allowed into the building early to wait in the warmth, and for another group of students to be kept outside shivering with no supervision.
I would put it to the school board EXACTLY like that and ask them why bussed students are receiving preferential treatment over walkers. That's what it is, no matter how they try and spin it.
Just for perspective, our school is almost all walkers- but the elemetary is attached to the middle school, so there is a LOT of bus traffic.
Even with 5th grade crossing guards and two streets blocked off and lots of moms around, there is a teacher each morning at BOTH intersections where the doors are AND our principal is standing outside in front of the main door EVERY morning, rain or shine to greet kids and parents and hustle them in the doors. We are not a particularly 'rich' school and its obvious that the staff is doing this instead of being inside doing other things. So it CAN be done, and your school ought to be doing it too!
Good luck!