You are not writing to children
Paper invites handed out at school depend on a small person carrying a piece of cardboard home and remembering to mention it. Some do. Others turn up at your door two families deep, because the invitation was read aloud at breakfast and the allergy question never came up.
The reliable route runs through the parents. Class groups, team chats, the swimming-lesson thread: that is where party arrangements for children are actually decided and confirmed. Which means your invitation is being read by an adult who is busy, who needs the pick-up time before anything else, and who will forward it to a partner to check the calendar.
Send that adult a page rather than a paragraph. The details stop being re-typed and start being read, and the reply form underneath collects what you need to run the party: who is coming, who is bringing whom, and what you must not put in front of them.