Your son knocked into me...,

[Silverobama]

Is the italic sentence natural?

Your son knocked into me when he was walking.

I was walking and suddenly a boy touched me with his back; he was looking at her mom talking to her and he was walking. Then I told the boy to walk carefully. His mom saw I was saying something to him but didn’t what, so she asked me what happened. I said the above sentence.

 

[Skrej]

I was walking and suddenly a boy touched me with his back; he was looking at her mom talking to her and he was walking. Then I told the boy to walk carefully. His mom saw I was saying something to him but didn’t what, so she asked me what happened. I said the above sentence.

The part in red doesn't make sense. You seem to have omitted a word.

Is the italic sentence natural?

Your son knocked into me when he was walking.

It's understandable, but the usual phrasing is 'bumped into me" or 'ran into me'.

Of course the latter can have a phrasal verb meaning of 'chance encounter', but context may clarify whether it's to be taken literally or not.