Duolingo is just a tool, I think. You can't rely on it entirely to learn a language. And especially you have to take an active role in learning when using duolingo. I'm using it to learn Japanese, and I think I'm picking it up somewhat decently.

But what I do is that I don't look at the word banks when translating, and when there's a listening activity, I don't look at the text on the screen. I just try to follow entirely based on what I hear. I always say the Japanese out loud, and I try to form sentences in Japanese by forcing myself to think in Japanese (as opposed to thinking in English and then translating the words into Japanese). And, of course, use other resources to figure out the nuances of the grammar and the vocabulary!

I think if you view duolingo as a way to get more practice with the language, it's actually a fantastic resource. You just can't rely on it as an exclusive learning tool

Also, the Japanese that's spoken in anime isn't really colloquial Japanese or really even the same Japanese that duolingo covers. Heavily exaggerated example, but it's a bit like asking someone to translate Shakespeare when someone is learning English. There will be some words that they can pick up, so your children might be able to get the gist of what's being said, but the tone and wording isn't really the same.

Not to mention, Japanese is spoken really quickly. Iirc, it's one of, if not the, fastest languages spoken, when measured in syllables over time. You would find better success with asking your children to translate if you find a Japanese speaking online personality who is known to speak slowly and clearly. Hololive is actually pretty good on this front