It should be free for your kids! It is free under 18 and free under 26 for EU citizens :)
kids 17 and under are free at museums and monuments in Paris. Eiffel tower is privately run and not included in that. I am not sure about the Marmottan which is also not in the museum pass. At most places they just walk in with you. The older one should have ID but will probably not be asked unless s/he looks 18. There are a couple of places you have to get 'free tickets' -- Invalides and Arc du Triomphe but at the others they just walk in with ticketed parents.
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