Hmm, well proteins aren't what is most essential for early brain development, that's fats. For example, omega 3 oils which come from grasses like flax, from hemp seeds, etc.
But if you're after protein, remember that your body can't use protein directly from your diet, it has to make protein from amino acids, the building blocks of proteins. We humans (animals) can't produce all those amino acids, which is whey they're called essential amino acids. You can get all the amino acids you need from your diet, and they can come from plants, fungi, bacteria, or animals. It's possible to eat other animals for nutrients, but to break down their proteins (body parts) into amino acids we can use, takes a lot of resources, and does damage to our bodies. Only then can we assemble the amino acids into the specific proteins our bodies need to live, grow, repair, etc. It's much healthier to get our amino acids directly from plants rather than eating other animals that have done so.
So, no, there's no reason for children or anyone to be eating anything but plants for their nutrition.
Perhaps you were mis-remembering what you read in some article somewhere... or maybe the writer of the article wasn't very educated on nutrition.
I put together this graphic to help people understand:

And I wrote this post, which contains all the info anyone could need on this topic.