Hi, positive could be praising a child for helping their friend put on their shoes or sharing buckets & spades in the sand pit. The praise aims to 'condition' children into repeating 'wanted' behaviours.
Negative reinforcement could be a time where a child snatches a toy from another child and no action is taken to help them understand what they did and the hurt that was caused.
How Children Learn by Linda Pound on amazon.co.uk
Best wishes