90p tickets are only available online, they are £1 if bought at box office. They're for selected Saturday morning screenings. Adults and kids can both get in for 90p-£1 per head, though adults have to be with a child in order to get the offer. To find the tickets, visit the Movies for Juniors offer page, then choose your film and cinema.
Though you can buy £1 tickets at the box office on the day, you may want to book as these tickets can go quickly. As it's only for selected screenings in selected cinemas, check your local Cineworld's participating before you go. Kids' tickets are valid for 14s and under.