The 24-hour clock is the standard time-telling system in many countries and organisations, including the British Army and the UK's railway network. However, because of the widespread adoption of the 12-hour am/pm clock for other purposes, many children never learn how to tell time on a 24-hour clock. Provide a worldly alternative to the 12-hour clock by teaching kids how to read a 24-hour clock.
Set two clocks -- one 12-hour clock and one 24-hour clock -- to the same time in the morning. Place the clocks next to each other so that they may be compared side by side. This will give children a basic knowledge of what times on the 12-hour clock correspond to the times on a 24-hour clock.
Explain a 24-hour clock as an "extension" of a 12-hour clock, rather than as a completely different system. Explain that when a 12-hour clock reaches 12:59 and resets to 1:00, the 24-hour clock keeps going to 13:00 and onwards.
Teach children that an easy-to-remember way to read the clock is that any time past 12:00 -- say 14:32 -- on a 24-hour clock, is in the afternoon. Teach that the larger the hour number on the clock, the later it is in any given day.
If the children have a basic understanding of addition and subtraction, teach them that they may subtract 12 from the hour of any time past 12:00 on the 24-hour clock to find its 12-hour pm counterpart. For example, 15:34 minus 12 hours is 3:34 pm.
Make a game out of the two clocks placed side by side. Cover the 12-hour clock with a piece of construction paper or fabric and ask the kids to guess the time on the 12-hour clock by only looking at the 24-hour clock beside it. When they have a good grasp of the concept, cover the 24-hour clock and have the children guess its time based on the 12-hour clock.