Hannah, I don't see anything on that page about age limits either, so I'd call the city number on the notice before you load up the kids. These giveaways are often first come, first served, and kids tend to get priority when supplies run short. Adults might be fine, but I wouldn't count on it. On sizes, most of the ones I've seen handed out are adjustable youth and adult models with sizing pads, so a pair of kids who've outgrown theirs should be OK. Probably not a huge selection, though, so I'd get there right at 4 if you can.
Since you're already going, a quick fit check: the helmet should sit level, about two fingers above the eyebrows, with the straps forming a V under each ear. A lot of helmets get worn too far back on the head. Doesn't help much there.
You're right about the light. Sunset this week is right around 5, and the Greenway gets dark fast once the sun is behind the trees. Front and rear lights make a big difference 🚲 I ride the trails a lot in the evenings and have had a few close calls with people dressed in all black and no lights. Bring a flashlight for the walk back to the car too.