Not sure to understand the question but if you remove the WG from the SELECT then you're going to see WGAlert for each PLA children and the WG default member. What would you like to see if this is not what you want?
[edit from Peter comment] you might try that calc. measure :
with member [alerts] as
count(
filter(
[dim_ProductModel].[PLA].currentMember * [dim_ProductModel].[Warranty Group].children,
[Measures].[AFR] > [Measures]. [Nonconformance_GPT]
)
)