That is because in Romanian the adjective comes after the noun, and not after, like it does in English. It is not incorrect to use both versions, but saying 'noi copii' instead of 'copii noi' is very unnatural to the native speaker.
In most of the cases, you can write the adjective before the noun, but it takes the noun's article (eg.: "Copilul mândru a luat note bune" / "Mândrul copil a luat note bune"). But it depends. As a native speaker, I consider "Tu ai doi noi copii" correct, even if it's less used