I'm not sure why we all started talking in the third person to our children but I'm here to tell you that you don't have to! And it's better not to!
For some reason, referring to ourselves as Mum/Mummy seems to have become the norm, e.g. "That's mummy's, don't touch" but that's not how we usually talk. We'd never say that to a friend! And seeing as we're teaching or children language from scratch, there's no reason to talk to them like that either! It's important that we model the correct sentence structure for children so that they know how to refer to themselves and others when they learn to speak in sentences.
Ever heard a child refer to themselves in the third person, e.g. "Sophie's teddy!" or use incorrect pronouns e.g. "Me hungry"?
This is possibly because they haven't heard frequent, proper models from the adults around them. Your child will learn how to use pronouns earlier and quicker if they hear them used properly by you.
Children are smarter than we give them credit for. They know that we don't speak to other adults in the same way we speak to them when we use mother-ese (baby talk). Speaking to children normally, with the respect we'd give another adult, not only shows them the correct way to use language but can also have a positive impact on their behavior. Saying "I won't let you hit me" instead of "don't hit mummy" instantly connects you to your baby person to person (It's you and I) to clearly set your expectations.
"but I'm worried my child won't know what to call me!". TRUST ME. They know you're mummy. And when they start babbling 'Mama' you're going to be lighting up like a Christmas tree!
It can take practice to make this change and you will likely still have the odd slip! I know I do. But give it a try!
Watch some examples here