Could an interior designer be housewife as well?

Yes, but if you want to make any money at it, you need to be super driven, at least somewhat decent at sales, and also have at least part-time childcare assistance (that is, someone who is neither you nor your partner). If you don't have all three of these things, it will just be a hobby and not a source of income.

The challenging thing about interior design in not the design part--it's the sales part (generating new clients), which means you have to go out and pound the pavement looking for new clients, which means that you have to spend some of the time outside of your house without your kids in tow.