1. Question: Why would tefillin that a child wrote be invalid because since he can’t put on tefillin yet the Torah says he can’t write them either but if a child shechts an animal it is kosher on a Torah level?
A. The Achiezer (3:82:12) pointed out that Tefillin is a new obligation everyday but the prohibition of eating a carcass which obligates one to shecht properly is one obligation per animal that can last for a long time, even after the child grows up.
B. By tefillin the sofer makes the tefillin kosher by following all the halachas of making tefillin. But by kosher meat the shechting isn’t what makes it kosher, it is the Torah permitting the animal and the shochet is just doing the action of shechting.
A2a. A child isn’t able to put on tefillin today so he can’t be a scribe for that mitzvah which is renewed every day but a child who shechted has ramification for the future since the meat can be eaten after he is bar mitzvah therefore the slaughtering is a good slaughtering since the same mitzvah can be used for when he is bar mitzvah.
A2b. By tefillin since the whole mitzvah is dependent on the person than if the scribe is a child then the mitzvah was not created since he is obligated in mitvos yet but since the Torah is what makes the animal kosher and the slaughtering is just an act to allow the Torah to make the meat kosher then it makes no difference if a child did it as long as it was done kosherly.