I’m going to punt here.
If the other two kiddos also live with your former husband half the time, I would think he could use them on the FAFSA…assuming he provides the majority of their support as well.
But because this is not going to align with his tax return (which he will need to link to the IRS Data Retrieval tool), and will result in Pell eligibility, it is highly likely that this will be selected for verification. He will need to be able to document that he actually provides more support…not an equal half, but MORE for this kiddo, and likely the siblings. Otherwise, for financial aid purposes, they really are not members of his household, they are members of yours.
Hoping someone else has an opinion about this…because this post is based on my understanding…and could be wrong.