Forgetting and feeding a vegetarian child meat is one thing, deliberately feeding a vegetarian child meat is another. If your parent/aunt/sibling comes to you and says "I'm so sorry, I completely forgot and offered junior a bite of my bologna sandwich without even thinking. I promise I'll do my best to remember he's a vegetarian from here out", then no problem , forgive and forget. If however, you find that people are feeding your child a food that you, the parent, have banished from the options on an on-going basis, then you need to decide how many times you will allow this to happen before you no longer allow that person to be with your child unsupervised. And state it: "If this happens again, then you will no longer be allowed to take Junior to the park on Saturday mornings without me/I will find another babysitter/I will take him with me when I go to the dentist." Full stop.
As for telling friends and relatives your rules for your child, you're should be clear and committed - "I/We do not eat meat in any form and therefore, neither does Junior. If you have any questions about what might be acceptable call or text me before you feed it to him."
People will come to admire your resolution.