Answer: Some Words You May Not Know
Explanation: "Solemn" is a bit uncommon. To find out what it means, you should use context clues (using words that relate to it to figure out what it means). You can note that the children are still, that it is a serious moment. Also, it says "They felt very glad, but also solemn." When the word 'but' is being used, it usually means that the next thing stated will be the opposite.
Glad, but (opposite)
Using these clues, we can determine that the word solemn means serious and sad. You can use context clues like this to help you understand what other words mean, too.
Hope this helps!