Nous on sera les bons

This is a mistake in french that many people say, it is just a repetition that isn’t supposed to be there. You are right, we should use

Nous serons les bons

EDIT

However, this could mark a certain tone in the sentence, like

Nous, (pause) on sera les bons

This can be like a heroic tone

This is a classic case of French redundancy, and it is very common in informal speech. It serves to emphasize the importance of who is doing what.

What’s confusing you is probably the lack of a comma after “Nous.”

“Nous, on sera les bons”

sounds perfectly colloquial, the same way you could say:

“Moi, je serai le meilleur.”

or

“Toi, tu seras génial.”

Dislocation is a very common process in spoken french; it’s probably more common in the singular 1st and 2nd person. We can have both dislocation toward the end of the sentence or the front, as is the case here. Front dislocation usually acts (as cccg03 notes) as a form of strong topicalization in French. You could easily analyze this as a contracted form of “c’est nous qui serons les bons,” where the topicalization is even more explicit.

“Nous on sera les bons” is a very common way to tell, especially from a group of kids splitting before starting some game, “Our side, we are going to be the good guys“. Technically, the grammar rules tell there should be a comma between “Nous” and “on” but this is no big deal, especially when transcribing a kid speech. Doubling the pronoun is there to insist about who is going to be the good guys.

It might be followed by Et vous, vous serez les méchants, i.e. “And you’ll be the bad guys“.

While technically correct, Nous serons les bons is far too formal to be used in such a colloquial situation, same for the unrealistic Nous, nous serons les bons.

Nous, on sera les bons.

This is a challenging situation with redundancy of subject. we (a share of total) *with a comma is better in good French. we shall be the best. (We are the best)

on sera les bons We shall be good (it is the same expression, but the entity is not defined. If you focus on us, gives you a enthusiasm expression. This is very verbal)

I do it. > it’s not suffisant.

I (Me), I do it. > it’s most.

It’s just a precision in the challenge.

Other example with redondance but the challenge can be a very bad interpretation in relationships (in business for example)
Entre vous et moi, vous, vous pouvez le faire.

Between you and me, you, you can do it.
It’s not (between ourselves) there’s more accentuation about capacity to do something and I (me) can’t do it.
Hawever, if you talk about a challenge with mockery or laughter, so, you have the capacity to do something but you don’t do it. we say with…irony.

It’s very classic in French to put accentuation into subject (nous) in speaking in place of the cause. That’s put in front the capacity and the implication of subject. That’s more “fine”.

Regards.

Double subjects are very common in spoken French: moi je, toi tu, lui il, nous on, etc. Why not? Conjugated verbs, in speech, don’t sound very different: je mange, tu manges, il mange, ils mangent are said the same.

Clarification

There’s a superiority with repetition.
A priority side.

Lui, il aura a une prime.

Him, he’ll be a bonus

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Eux, ils y vont. Nous, après.

Litteraly : Them, they go. Us after.

“On” is used far more frequently in spoken french to replace “Nous” with a conjugated verb. E.g. “On y va!”, instead of “Nous y allons!” which would be far too formal (using ‘Nous’ conveys a ‘stuck up’, snobbish or outdated kind of tone in spoken language)

However , “Nous” is still exclusively used when answering questions like “Qui sont les meilleurs ? (Who are the best ?) ” Answer: “Nous!”

The sentence “Nous On sera les bons” is a juxtaposition of the 2 concepts , and is very widely used in spoken french, especially in child speak. It makes even more sense when knowing the context of the sentence, from the ‘Petit Nicolas’ books.

The lack of a comma between ‘Nous’ and ‘on’ is intentional and typical of child speak, with very long sentences made without punctuation to convey a child speaking fast and almost running out of breath as he tells his story.