Simon, 3yrs, has two parents, Sam and Sally. They are a couple.
Daisy, 17yrs, has two parents. Sam (Simon's dad) and Debbie.
Debbie has a husband and they took Daisy to Greece last year. So, Daisy has a holiday paid for by one parent, with one parent and their partner in attendance. Because Daisy doesn't live with both her parents, so she won't ever go with both of them (anymore). Sam couldn't afford to pay for Daisy to go to Greece, irrespective of the fact it's not his holiday to go on.
Sally wants to take Simon to Spain. His holiday is paid for by his aunt, with both parents in attendance, because they live together. Sam couldn't afford to pay for Simon to go to Spain.
Daisy goes on a holiday with zero contribution from her father, Sam. Simon goes on holiday with zero contribution from his father, Sam.
Sam only goes with Simon because he is being gifted a ticket by Simon's maternal family, because he lives with Simon's mother. It's not just because he's Simon's dad, because if he was Sally's ex, he wouldn't be getting a ticket.
Sam is treating his children equally. It is Sam himself who's getting the special treatment, not Simon. The idea that Simon can't have his father, whom he lives accept a free ticket to Spain because he's also Daisy's Dad, when it's perfectly fine for Daisy to go away with the adults in her house, is ridiculous. This is solely because Daisy's parents don't live together. They fucked that up. Not Simon, or Sally. Sally shouldn't have to turn down amazing free holidays with her husband and child, from her family, because Debbie and Sam can't make a marriage work, and had a child.
That's what happens when people split up and already have kids. The kids have two households, two lives, and they are not supposed to be mirror images. Sometimes this works in Daisy's favour and sometimes it doesn't. It's not for Simon to live a weird second rate life skirting around Daisy. He's an equal. He gets to be prioritised too.