There's a book in our van more often than not called I Really, Really Need a Wee.
The joke is in the title. A child on an outing, badly in need of a wee, finds that everything and everyone gets in the way. By the end of the book the wait has become enormously funny, and the reader, without anybody pointing it out, has been shown that waiting your turn is survivable.
Karl Newson wrote it and Duncan Beedie illustrated it. Reception and Year 1 classes we've sent it to know it straight away. Parents who have read it at home sometimes quote lines back at us when we meet them.
What we like about this book is that it takes one clear problem, sets it up, pays it off, and stops. No padding, no message, just rhythm and laughs. That is harder to do than it looks and it's the thing that gets children asking you to read it again.
If your school would like a class set of I Really, Really Need a Wee, or any of Karl Newson's other titles, mention it on the registration form.