This young woman is wearing non-traditional clothing because she's going to highschool in Luang Namtha town.
Unlike the other women, she also doesn't pull out her eyebrows. When I asked her, she said that if she goes to school in traditional clothing and removes her eyebrows (usual for Yao Mun women), she feels she'll be laughed at and looked down upon by the other students. There is a bit of a pecking order in the province (and the country) with some groups being seen as having lower status then others, with groups like the Yao Mun being seen by many others as close to the bottom of this pecking order.
No, the child is not her daughter.