My son is 6 now, and he has loved these two books since he was about 3:

The Boy Who Loved Math: The Improbable Life of Paul Erdos - https://www.amazon.com/Boy-Who-Loved-Math-Improbable/dp/1596...

Bedtime Math: A Fun Excuse to Stay Up Late - https://www.amazon.com/Bedtime-Math-Excuse-Stay-Late/dp/1250...

We also started this book recently, and he has really enjoyed it so far:

Women in Science: 50 Fearless Pioneers Who Changed the World - https://www.amazon.com/Women-Science-Fearless-Pioneers-Chang...

that's a shame. (especially since I just bought it.)

Maybe I should have got this instead? https://www.amazon.co.uk/Women-Science-Fearless-Pioneers-Cha...