Object - Assignment problem (Linq to sql classes)

Hi All, I am having following issue.....
here is my code..

public int Add(BWS.DB.DAL.ProductMaster product, string strColorSizeIds)
{
try
{

ProductDataDataContext objDataContext = new ProductDataDataContext();
if (strColorSizeIds.Length > 0)
{
string[] strArrayColorSize = strColorSizeIds.Split('_');
for (int i = 0; i < strArrayColorSize.Length; i++)
{
product.ChildProducts.Add(new ChildProducts()
{
BarCode = product.BarCode,
CanInventorize = product.CanInventorize,
ColorId = Convert.ToInt32(strArrayColorSize.Split(',')[1]),
SizeId = Convert.ToInt32(strArrayColorSize.Split(',')[0]),
CompanyId = product.CompanyId,
Description = product.Description,
haveColorSizeMatrix = false,
id = 0,
ImageUrl = product.ImageUrl,
isSerialNoReq = product.isSerialNoReq,
MinInventoryLevel = product.MinInventoryLevel,
ParentProdId = product.id,
ProductClassId = product.ProductClassId,
ProductCode = "0",
ProductFamilyId = product.ProductFamilyId,
ProductSuppliersCostGrids = product.ProductSuppliersCostGrids ,
SellingPrice = product.SellingPrice,
IsActive = product.IsActive
});
}
}
objDataContext.ProductMasters.InsertOnSubmit(product);
objDataContext.SubmitChanges();

return 1;

}
catch
{
return -1;
}
}

ProductSuppliersCostGrids is my class in same datacontext and I set the relationship it to the productmaster class.

My problem is when I used above code( ProductSuppliersCostGrids = product.ProductSuppliersCostGrids) to insert the same suppliers list to childproducts, It's only added to first child product, not even in the main product and other child products..

Why?????????????