Eliza is an traditional nickname now current/trendy again, so the first names that sprang to mind for me were similar Nell, May, Annie and Kitty (could be Helena/Eleanor, Mary/Maria, Anna and Catherine if you wanted longer versions for the BC).
Otherwise I think other retro/vintage names like Cora, Freda, Iris, Lillian, Florence, Clara, Vivien, Louisa, Josephine etc sound nice with Eliza.
For boys I would either go very timeless (so suits any sibset) like Edward, Thomas, James, Alexander, Charles, George, William etc
Or more traditional sounding - John, Robert, Frederick, Alfred, Henry, Phillip, Hugh, Wilfred etc