A lot of these get upvoted because kids are cute, not because the joke is particularly funny. There are various great subreddits to post cute things that your kids say, but r/jokes is is not r/aww.
Also, Conspiracy Alert: I think people are just using the "my kid told me this" line so they don't have to take responsibility for the corniness of their joke. Have some pride OP! If a joke is good enough to share with thousands of strangers, it should be good enough to take credit for.
TL;DR: A bad joke does not become a good joke because a child told it.
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