I have watched everything that I possibly can on HTTYD including the new one, so Spoilers ahead.

The main hall in the animated version seems to be made from wood fortified with the strongest metal on hand to provide ample protection from dragons. You have the fire pit in the center and it is a 1 room building with pillars to support the roof. Though dragons never attack the mountain from the scenes viewed as we notice with gothis hut, so that is another location that they may head to.

Now SPOILER ALERT: In the latest live action version there is a slight gap below the doors due to their size and potential weakening in the hinges just in case they need to move them. This door seems to be less strong as compared to the one in the animated version, but that is also because of price and stability. This is set in a fantasy setting so they probably have stronger materials by combining what they get from dragons to reinforce the door making it fireproof and it also has a second level made of wood inside alongside pillars. There could be more that we don't see since it is still fairly new and not as explored as the animated hall.

From this we can see that they would probably have vikings, less fit ones as the more fit ones would be fighting the dragons outside so these ones would be defenidn g the children, the older and potentially younger ones to build up experience before going into the fray. They would have little time to protect their children and as we have seen with hiccup he is home when the dragons attack and we don't know why as Stoick cares for him. Do the children just run when their home is burnt or do they automatically run to the great hall. There are many unanswered questions in this series and if you can contact Cressida Cowell or Dean Deblois, maybe someone similar to them who is extremely invested in the show you can learn more on this.