What do you think of me only serving vegetarian food at my son's first birthday party?
Why? Is your son only a veggie eater?
Haha not at all, I just want to try something new with my parenting.
If that is the food that you want to serve, then I say go for it. You should serve the food that you know your son likes, it is his party afterall.
There are some people who told me they can't come to my party since they eat meat.
If these are people you value having at the party, there's no reason why you can't have both types of food, if you wanted. It would be thoughtful for those in your circle of friends and family who prefer each type of food. It is a bit silly of them, though, to make such a big deal of it, as vegetarian dishes can be quite tasty and enjoyed by everyone.