There's a difference in just stating, "oh John, he's my first, Amelia is my second" etc, and "John should be prioritised, he's my first". OPs posts lean towards the latter.
A child might not be able to spend christmas day with both parents at the same house but a child must be invited to his or her dad's house every year whether the child chooses to go or not. It's about it being an open house for the child and unhindered access to their father.
At 7, the child isn't choosing anything. It's down to the parents. I don't understand this "as long as mum says it's ok, he should be going" either? It's not her call, she's not the sole decision maker.
There are two households. Two rents, two sets of alllll the bills, all the to-ing and fro-ing between the two. The child benefits from two households (for arguments sake) 50% each. The other children benefit from one household 100%.
The households are not identical. That's what happens when parents split up. That's life. Caused by the decisions of the parents. Maybe the child's mother provides 50% of a high level lifestyle and dad is skint and his 50% is of a lower level. If there are children 100% resident in mum's home, they will receive 100% of her higher level, and should not be penalised simply because that's their only household, and another child has two households.
Also, if mum's/dad's household is significantly higher level, not due to them, but their partner, then it's not either biological parent's place to start making comments about their child not receiving the same treatment, because the "benefit" comes from a person who is neither of the child's parents. And yes, the child that is that partner's child does, and should, receive the benefits, and no, the non biological child isn't entitled (no matter how much the parents seem to think they are!) and if the partner chooses to share those additional benefits with a non biological child, that's obviously great, but it's their choice, and their choice alone.