According to my experience, kids will only wear casual wear during the party day. Or they will wear Chinese style cloth if the party conduct near Lunar Chinese New Year. Therefore, denim trousers and T-shirt is OK !
On the other hand, if you want to take photos with your kid, you may take before school or when you pick them up. Because no parents are allowed during the party. Teachers will take photos with the birthday kids and will give them a hard copy of the photo. But only for the birthday kid !