Actually, humans are animals.
Humans are omnivores. Omnivores are creatures capable of consuming both meat and vegetation. We have sharp canines for a reason.
Vegetarianism is a purely ethical decision, and its not a bad decision, I'm not saying that at all. But there is no physical reason that humans cannot consume
moderate amounts of meat.
I do agree that there is no need to be a glutton and eat three megawhoppers for dinner. I personally don't eat a lot of meat, but I know that it can be a viable source of minerals and protein, and I do enjoy the taste of it.
I am also anti-abuse, so my consumption of meat is reserved mostly for seafood and other animals that must be caught, giving them a fighting chance. Pork, chicken and beef all suffer horribly before they are finally released from torture, and I can't support that at all.
As for when to feed your child meat... whenever they are capable of eating it. I'm sure there's a resource for that, but I'd say once they can handle semi-solid foods like fruits and vegetables and pastas, then start them on a little meat every now and then.
Proportions are what is important. Also, if you are vegetarian or vegan, you absolutely
must make sure that your child is receiving enough protein. Children need more protein than adults, as they are growing and developing. What is healthy for an adult vegetarian could very well starve or cause developmental issues in a child.