I smell potential trouble.
This is what you do -
1. Go to the local cops, make a report that you saw 2 unattended children, names, date, time, witnesses. Cops will probably notify child protective services, but it won't be your business any more.
2. Send your neighbor a certified mail [very important!] letter stating that you saw her grand-kids[?] on such and such date, and they were unattended for x number of hours and you think it's unsafe for their well-being to run around unattended. Save the receipt and a copy of the letter for your records.
If the kids show up unattended in your yard again, call the cops right away.
Zai Gezunt