Start with the guest list, not the theme
How to build a children's birthday guest list before you book a venue: ages, adults, siblings and the children you are actually inviting.
Write the names down before you fall in love with a photograph
The venue follows the children you are actually inviting. A room full of four-year-olds needs a different building from a room full of eleven-year-olds, even if they share a birthday month and a love of dinosaurs. Themes are easy. A guest list is a set of real children with real ages, real parents, and a real chance of saying no.
Start with a piece of paper, not a Pinterest board. Column one is the child. Column two is their age on the day, not the age they were at Christmas. Column three is whether a parent will stay. That third column changes the size of the party more than the theme does.
Count the adults as guests
A party of twelve children can quietly become twelve children and fifteen grown-ups if every parent stays. Some venues price that. Some simply run out of chairs. Toddler parties and farm parties are adult parties in disguise: the grown-ups are not dropping off, they are the supervision.
Ask, when you book, what 'one adult included' means. Is it one per child, one for the birthday child, or one for the organiser? Put the answer in the invitation. Parents who turn up expecting to be fed, and parents who turn up expecting to leave, are both reasonable. They just need to know which party they were invited to.
The class list is not the guest list
You do not owe a party to every child in the year. You do owe honesty if the school rule is 'all or none'. If that rule applies, pick a venue that can take the number, or pick a smaller celebration and say so. A soft-play hire that caps at twenty will not stretch because you added six more names on Thursday.
Leave a margin of two or three. Someone will be ill. Someone's grandparent will be visiting. Do not book a minimum of twelve when you only have ten maybes. The deposit is the price of pretending the list is firmer than it is.