Excellent question for a six year old! Actually, a good question for a 20 year old!
One explanation is a bit more complicated. Your son thinks that after the Christmas season the company is worth more. For example, they might have turned $10 million of goods into $20 million of cash, which increases their assets by $10 million and is surely a good thing. However, that's not the whole picture: Before the Christmas season, we have a company with $10 million of goods and the Christmas season just ahead, while afterwards we have a company with $20 million cash and nine months of slow sales ahead.
Let's say your son gets $10 pocket money every Sunday at 11am. Five minutes to 11 he has one dollar in his pocket. Five minutes past 11 he has 11 dollars in his pocket. Is he richer now? Not really, because every minute he gets a bit closer to his pocket money, and five past eleven he is again almost a week away from the next pocket money On the other hand... on Monday, he loses his wallet with $10 inside - he is now $10 poorer. Or his neighbour unexpectedly offers him to wash his car for $10 and he does it - he is now $10 richer.
So if the company got robbed in August with all stock gone, no insurance, but time to buy new stock for the season, they lose $10 million, the company is worth $10 million less, and the share price drops. If they get robbed just before Christmas sales start, they don't make the $20 million sales, so they are $10 million poorer, but they are $20 million behind where they should be - the company is worth $20 millions less, and the share price drops twice as much. On the other hand, if there is a totally unexpected craze for a new toy going on from April to June (and then it drops down), and they make $10 million unexpectedly, they are worth $10 million more. Expected $10 million profit = no increase in share price. Unexpected $10 million profit - increase in share price.
Now the second, totally different explanation. The share price is not based on the value of the company, but on what people are willing to pay. Say it's November and I own 100 shares worth $10. If everyone knew they are worth $20 in January, I would hold on to my shares and not sell them for $10! It would be very hard to convince me to sell them for $19!
If you could predict that the shares will be worth $20 in January, then they would be worth $20 now. The shareprice will not go up or down if something good or bad happens that everyone expects. It only goes up or down if something happens unexpectedly.