I've been reading the version Graded Readers version of the biography of Martin Luther King Jr. by Alan C. Mclean.

It's a good book which shows a good background about what had happened with black people in the USA in the past century and what was the background that allowed Martin Luther King to becoming ine a voice for them.

In the first chapters, the author explains that Martin Luther King was a black minister from Alabama in the sSouth of theern USA who grew up in Georgia, Atlanta in the middle of all the racism which existed in those times.

'the south of the USA' would be understood fine, but it's a bit long and awkward. Most people would say 'The Southern USA' or even 'the Southern US' in this context.

'all racism' = all racism that ever existed
'all the racism'= all the racism that existed in this time and place

For sureDefinitely, it must have been a hard childhood.

This isn't bad English but using 'for sure' in this way is very casual/conversational and isn't usually seen in formal writing.

Those experiences and events he lived through were the engine which motivated him to fight all his life against racist laws ahead in his life around allcross the sSouth in theern USA.

I wasn't sure if 'all his life' was exactly what you meant here. Sorry if I got that wrong.

An event he lived through in his childhood which iwas quite unfamiliar to me but caught my attention was that he had a friend in front of their house with whom used to play outside in front of his house.

By putting the sentence in the order it was in, it implies the friend lives in front if you the house. You can't say in English someone lives in front of your house (unless they're living in a tent in your front yard or something like that), usually you would say they live across the street.

One day the friend's mother decided to separate them and never moreagain allowed him to play with Martin.

'Never more' is a very old fashioned turn of phrase. Though you may also see it if people are trying to sound poetic.

I can remember very well when he asked his mother why the mother of his friend didn't allow him to play with him.

I'm still reading the book, and I'm enjoying to read it.

Withouth doubt this is a good gGraded rReader.