Hi Celticmum and welcome
I was in your shoes just over a year ago.
that this is the healthiest lifestyle for your daughter, we don't make decisions that affect our children on a whim, we do what we believe in our hearts is the right thing with our childrens' welfare being a huge priority. Therefore, don't feel that you have to explain yourself or your decision to anyone, especially if they are 'going on' about things like protein, iron and calcium.
You can tell who is genuinely interested and wants to know what you know about nutrition - and that's fine, talk away. You might even convert someone!
But those who are 'going on' about it and who you feel are just being antagonistic or who really aren't interested in learning anything... I would just say to them "I am happy and content in my own knowledge of nutrition and I'm happy about the choice we've made to be vegetarian" and leave it at that. There's no point in wasting your breath and energy on people who have closed ears and hearts. You don't have to convince anyone that your daughter is getting enough of anything in her diet.
If you come across someone who obviously does want convincing, tell them you'll email them the link to some good information on the topic. This also means that you don't have to enter into a debate about the pros and cons of animal source iron versus plant source iron... (been there, done that).
Alternatively you can just focus on the ethical issue. In a few situations I've found that once I say I chose to be vegetarian for ethical reasons, the omni mums can get cagey and they stop asking questions real quick for fear that I might start preaching.
Good luck, I know it's not an easy road to travel but you generally only have to do it the one time. Once the other mums see that you're not about to crumble under their peer pressure and seeds of doubt
they'll get over it, hopefully they'll come to respect your choice and you can all move on.