Question

we watched for a few minutes; then as trade slackened we went over. plzz explain this line

Answer 1

Here,
It means that the narrator was watching the kids polishing the shoes at the square.
It was a brisk(quick)business.
The narrator didn't want to disturb to them while they are doing their business.
But as all the customers went away,the narrator thought that it was the time to go and talk to them.

Here 'Trade Slackened' means the business is over.

Thanks.