I'm not too sure as apart from now, I have never heard of a trolley tipping over before at all. If it was a regular occuarnce and if the kids were seriously injured because of it, then you may have a case. However, unless the trolley you used was a faulty one, I think it may have just been a one-off incident.
I'm glad to hear that Charlotte is Ok though. Have the docs said anything about a second x-ray in a month or so to see if there is any superficial damage at all, to be on the safe side? If not, I'd definitely request one, as from what you have described, that's one hell of an accdent!