DHS is actually following the State statutes (Title 17-A, p. 554 Endangering the welfare of a child) on this one (but might be stretching just a little, too). If you're the adult in charge of a child (under 16), any firearms and ammunition at the location where you're supervising/adulting? must be separated and locked away so that the child cannot have ready access to said firearms and ammunition.
Now, both DHS and the State statute (Title 17-A, p. 554) focus on you being the child's long-term care provider.
You're post indicates that you're may not be the long-term care provider, but happen to be at the location where children are at. No harm, no foul. Keep your handgun concealed and away from the children (unless the situation calls for you to use your handgun).
If you become a Sunday School teacher/helper and begin to provide regular care to the children, I would contact an attorney who would be better able to give direction on this.
Here's a link to the State statute: Maine Title 17-A