I agree with MissLydia. The thicker neck is usually female, and the one you fear may be a gander looks to me to be a female.
Some hatcheries are excellent at vent sexing, but not uncommon to get boys from feed stores that are sexed "female". Your pair are getting old enough that vocal pitch and behavior reliable to determine gender.
While I agree that most geese only bluff people about being aggressive, I respectfully disagree about a goose bite not hurting, not to mention being scary for someone not confident with animals. They may not have teeth, but can strike and pinch very hard and leave a pretty decent bruise. If safety is a concern, find a light stick the length of child's arm for child to use like a walking stick, and tell child to use ONLY if necessary. Even the most aggressive gander during breeding season will respect a stick and give proper space.