If he mocks you, let it roll off your back. You're doing the right thing. You need to worry about putting food on the table. Not dressing the kids up for a fancy outing you're not even taking them to. All you're discussing is the clothing. Not what kind of mother you are. Not your financial situation. Just the fact that they don't have these dressy clothes he needs them to wear for his function, and he'll need to provide them since you don't have them already.

Good Luck!!! Let us know how it goes.