I have a job as a sales copywriter for a business that mainly sells generic children’s toys, but during holidays like Halloween and Christmas, they sell toys with those themes/decorations. Is it permissible for me to write advertisements for these holiday toys?

Allah swt says what could be translated as:
“… Cooperate with one another in goodness and righteousness, and do not cooperate in sin and transgression. And be mindful of Allah… “
Al Ma’dah – verse 2

If you are not personally directly involved in promoting, endorsing, and advocating for the sinful type of advertisement, then you should be fine to work within this company even if it manufactures products with themes that do not align with our Islamic values.
Of course, if the business you work for, is established to only promote these types of holiday toys, It would be impermissible to work with this company.
Allah does not accept our direct involvement to commit sin or help others do so.

In summary, If you are promoting the same theme of toys but turning into seasonal toys with colors that pertain to these holiday occasions, then you should be fine to do so. However, If you are directly promoting toys that indicate shirk, such as the son of God, wives of God, and figures associating partnerships with Allah, then this type of job is not allowed.