Is it permissible to tailor children's clothes for occasions such as weddings, birthdays, and school parties, which often include music and singing, or is it forbidden?

If the clothes are specific to certain occasions and are only used for them, then the ruling on sewing them depends on the ruling of the occasion itself. What is permissible among occasions, such as weddings and school parties, makes it permissible to sew clothes for them. What is impermissible, such as birthday celebrations, does not make it permissible to sew clothes for them. The rule is that means take the rulings of their ends. As for what may be associated with the occasion in terms of religious transgressions, such as music, that is a matter external to the occasion itself.