There's an awful lot of ground being covered here isn't there? Now were do I begin:
Firstly, I think it's far more cruel to, say, give a toddler a caged bird, and condone birds being caged, than it is to let a toddler run after some pigeons who are, lets face it, free to make an escape. I mean how many toddlers have you seen proudly brandishing a pigeon that they have just swiped from the park?
But lets take two toddler/pigeon situations: Firstly a wobbly-on-their-feet 18 month old. Secondly a fast, loud and lairy three and a half year old. If toddler number one took a few steps towards the pigeons I would not say 'no'. Pigeons being sociable, scavenging birds, are used to close proximity with people. I would not see my little toddler as a threat to them. If toddler number two ran after them however, I may well say no. Being bigger, faster and louder, I accept that my toddler some capacity to frighten a pigeon and, most importantly, was old enough to know it.
But it's not simply to do with the balance of power, toddler versus pigeon, is it? It's also to do with your toddler's behaviour and what you are prepared to tolerate.
Well, children are naturally curious aren't they? It's how they learn. In my experience, children want to touch, get up close to, and interact with most living things (spiders and snakes excepted). All that pleading for pet rabbits,hanging round the farmyard stables etc. So isn't it natural that if they see pidgeons running around, getting up close to humans as they do, they will want to copy them and run after them too? I mean I'm an adult and I've been quite upset by mauranding pigeons coming after me! Just try having a picnic in a pigeon-haunted park. If you are going to judge pigeons on human terms, then pigeons are birds that misbehave. What sort of example do they set?
As for more general misbehaving, Jbr, Numbat etc, I can quite see your reasoning that kindness to all living things should start from the earliest age. Yes it should. Who can argue with that? And Jbr I can quite see why you would want to intervene in the incident you describe, though to be honest I would have been more concerned with the fact that a young child was running freely by the road. Pigeons would have been a secondary worry.
Which brings me to a final point. You are trying to teach your toddler/young child to do a million things properly: eat with a spoon, use a potty, say please and thank you, sit nicely, stand still, keep their coat and hat on, hold hands, not walk on the road etc etc.
To my mind, not running after pigeons comes rather low on the list of desirables. Unless you want to spend all your time telling your toddler off, sometimes you have to compromise and let the less important things go. Not for ever, but for when they can take it in better.
And that is all I have to say on the subject for now.