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I would definitely not agree on giving a "talk" to 5 yo kids. It is very difficult to keep their attention for more than a couple of seconds. Unless, you tell a story.

The topic I would choose is "counting", and being more specific "counting by ordering"

Example 1: Make a slide with 15 dots in random positions, ask them: How many points are there?

Then a second slide with 15 dots, three groups each of 5 dots arranged as in the face of a dice. Make the same question as before.

Then a third slide with 15 dots arranged in a rectangle (if they can multiply this is easy 3x5). again make the question.

The moral is that by ordering counting is easy.

Example 2: Make them walk in the room, and ask them: How many kids are in the room? But donĀ“t let them stop walking... -Do you want to stop?- then make them stop and ask them again... this should be much easier (even for an adult)

You can include some problems on counting (like those where you have to conun how many triangles are there in a given picture)... you know, there you need to be careful not to count more than once... or make them draw a couple of lines in a piece of paper, then mark the intersection points and when they're finished ask them: -How many points are there? -how many triangles?

... well I guess you got the idea.

wish you luck!