Starting at about 5 I was allowed to play on the sidewalk with other children on my street. Younger siblings were with us at times. We were not allowed to pass certain boundaries and had to ask a parent to come out and walk with us if we went across the street. We ALL knew these rules clearly. Ball bounced into the street go get mom. Want to go roller skate in Sally's drive way, go in and tell mom we're changing location.
I agree with the follow up to the original post, we've lost some perspective. Five year olds need adults, but they are not helpless. I walked to school in kindergarten.
It is easy to assume her action is out of neglect, disregard for safety, etc. But maybe in her mind leaving them to swing in this park is not different than running in the house to take a call if they're in her back yard. Thirty years ago neighborhoods were teaming with kids playing kick ball, tag, hopscotch and other games as their mothers viewed them from inside. I don't know if the dangers are more dangerous today, but they've always been there. I was instructed when playing in my neighborhood what to do if a person talked to me, never to go in anyone's house, not to approach a car, etc. So the dangers were there in the 70's. Our idea of how to protect and care for our children has changed.
On my street school aged children sometimes play unattended in their front yards. I don't feel comfortable letting mine join them, and don't imagine I will when they're older either. There are risks I don't want to take. And I am a mother of the 00's. That being said, at some point, I will have to allow my child to walk to a friend's house, school, etc. without me being a constant shadow.
It is entirely possible that the mom in the situation described considers this park directly across the street from her house as an extension of her yard. From the description, it sounds like they might be visible from inside her house.
I would approach her. A park is NOT part of her house and a three and five year olds need direct, hands on adult supervision in public places. As previous posters said, it is probably the law.