Add a child product to grouping one

That's how you add child product to grouping one.
Please bear in mind, that quantity isn't required and if used, cannot be a string - must be Integer (so there's no quotation mark)
Both products must exist before this operation.
mutation {
input: { sku: "SKU_GR_PRODUCT", childSku: "SKU_test2", quantity: 1 }
) {