"The kids, however, are less excited."
Translation:Die Kinder hingegen sind weniger begeistert.
I believe "hingegen" is a better translation than "dennoch" because the English sentence "The kids, however, are less excited" implies that someone else (not the kids) is more excited. Basically the "however" (dennoch) is used here as a more subtle form of "on the other hand" (hingegen). It's a small nuance, but Duo still wants us to learn it. I also made the same mistake you did.
Both "jedoch" and "hingegen" work well as a translation of "however" (and both are, among many others, accepted as an answer, provided the rest of the sentence is correct).
But you have to note that they use different word orders.
"dennoch" has a slightly different meaning. It refers to something that must have been mentioned before and rather translates to "in spite of that" or "nevertheless".
(I know there are contexts where "however" could be translated by "dennoch", but those are rare).
That's a tough one. Probably ou can see it as an exception, because the "hingegen" is directly attached to "die Kinder", so both together taking only one position.
But you can (and I'd propose you rather do that) forget about that and use the ordinary word order: "Die Kinder sind hingegen/jedoch weniger begeistert". Fortunately this is accepted as well.
The "main solution" (see above) is a correct, but not the most natural word order in German. Maybe it was chosen because it is closer to the English word order.
I think "Die Kinder sind hingegen weniger begeistert" (which is accepted) better. "Hingegen sind die Kinder weniger begeistert" is possible as well, but hasn't been in the database up to now, Added.
I could be wrong, but, to me, "allerdings" would connote more of an affirmation that the children are less excited if uncertainty been expressed previously.
"Die Erwachsenen sind begeistert, aber ich glaube die Kinder sind so weniger." > "Die Kinder sind allerdings weniger begeistert."
It is accepted. I think it is even the better solution. But the word order is different from English. Unlike the English "however", "jedoch" is not separated from the rest of the sentence by commas, but takes the ordinary position of an adverbial in German: "Die Kinder sind jedoch weniger begeistert" (and some less common word orders).