tosafos has quotes from rabbeinu tam that justas wepassul sta'm by a woman because whoever is not included in tying tefillin isn't included in writing, we also passul a woman from making tzitzi since she isn't included in wearing them. tosafos argues that the gemara implies the opposite since by sta'm we need a special drasha to assur her. how would r't answer tosafos question? perhaps we need special drasha only to exclude a yisroel mumar who is commanded in tefillin but doesn't do it. those who r exempt such as a woman, we wouldn't even need a drasha for. see mahratz chiyus that we even exclude a jew who doesn't believe in chazal, which we certainly need a drasha for.
l'maseh, the shulchan aruch 14:1 paskens like tosafos to allow a woman to make tzitzis, but the rama recommends being machmir for those who passul. it seems from the gemra that achild is also passul according to rabbeinu tam, since he is exempt from wearing tzitzis. however, the biur halacha suggests that the rama may not be machmir for rabbeinu tam, rather be machmir for the maharam that e exclude a woman from tzitzis with a specific drasha, but there is no rule passuling anyone who is not included in the mitzvah. the nafka mina would be tzitzis made by a child. we are machmir lichtchila based on pri megadim and derech hachayim (nesivos) who r choshesh for rabbeinu tam.