Before the 9 days began, I bought and washed some plain undershirts for my son. Can he wear them during the 9 days?
It is permitted for the child to wear them.
The Ashkenazi minhag regarding washing children’s clothing during the nine days is that we may wash their clothing between Rosh Chodesh and the week of Tisha B’av. For example, if Tisha B’av is on Tuesday, then it is permitted to wash their clothing and for them to wear them between Rosh Chodesh and Shabbos, but not after Shabbos. This year since Tisha B’av falls out on Sunday, we don’t have “the week of Tisha B’av”. Therefore, the child may wear the washed undershirt during this whole week.