I think this is a matter of approach. If you feel that the kids are darting in the street, not careful around animals, etc, then I don't think there's anything wrong with telling the mom "Hey, I've noticed your children running up to the dogs in the neighborhood and some of these dogs are not child friendly. I'd hate to see something happen to them (or the dogs) because they aren't aware of the danger." or "Hey, I was walking my dog yesterday and two of the children ran out in the street. There's a 'blind corner' and cars tend to speed. The streets in this neighborhood are not safe."
I don't think you can say "Hey, you let your kids 'roam free' and yelling at them periodically to come back inside is not appropriate. Aren't you aware of the dangers?"
One is giving a friendly 'heads up' that the kids are not playing safely and another is questioning her parenting skills/techniques. The latter is rude.
I also agree with those that say you should introduce yourself first and welcome her to the neighborhood before you decide to tell her how much danger her kids may be in.