I am a Geography teacher at school, I have a problem with the children as they always question me "what proves that the earth/world is rounded?". is there anyone who can help me answer this question clearly?

You can explain to your students with an example:

Explanation:

If you’ve been next to a port lately, or just strolled down a beach and stared off vacantly into the horizon, you might have noticed a very interesting phenomenon: Approaching ships do not just “appear” out of the horizon (like they should have if the world was flat), but rather seem to emerge from beneath the sea.

But—you say—ships do not submerge and rise up again as they approach our view (except in Pirates of the Caribbean, but we are hereby assuming that was a fictitious movie series). The reason ships appear as if they "emerge from the waves" is because the world is not flat: It's round.

If you stick a stick in the (sticky) ground, it will produce a shadow. The shadow moves as time passes (which is the principle for ancient Shadow Clocks). If the world had been flat, then two sticks in different locations would produce the same shadow:

Imagine the Sun's rays (represented by yellow lines) hitting two sticks (white lines) some distance apart. If the Earth were flat, the resulting shadows would be the same length, no matter how far apart you place the sticks.

         But they don’t. This is because the Earth is round, and not flat

Sticks placed at distant locations will throw shadows of different lengths.

since there isn't any suggested answers i will give you the reason and hopefully you can figure it

south africa, or rather all of africa, has not been in the location in which it is nowadays throughout the geological past. after the super-continent pangaea started to break apart, gondwanaland, in which was africa, moved further south, and parts of it came to be very close to the south pole. because the climate in this part of the world is very cold due to the very dispersed sun rays in there, what is now south africa, started to freeze, and the conditions were right for the formation of glaciers. the glaciers existed up until the period when gondwanaland started to break apart and what is now africa started to move towards north, sp it warmed up and the glaciers melted.