Where to buy good and cheap clothes in Beijing?

Do you mean kids clothes? If so, you could find them at any department stores in Beijing. Along the Beijing Wangfujing Dajie(https://www.travelchinaguide.com/cityguides/beijing/shopping/wangfujing-dajie/), there is also a special department stores for kids. For the cheap but not so qualitied clothes, you may consider the Silk Street, Beijing(https://www.travelchinaguide.com/cityguides/beijing/shopping/xiushui-street/) and these markets around Beijing Zoo.