```python
def digit_word(digit: int) -> str:
    """
    Convert a single digit (0-9) to its English word representation.
    
    Args:
        digit (int): A single digit between 0 and 9.

    Returns:
        str: The English word representation of the digit.
    """
    if not isinstance(digit, int) or digit < 0 or digit > 9:
        raise ValueError("Input must be a single digit between 0 and 9.")
    
    words = ["zero", "one", "two", "three", "four",
             "five", "six", "seven", "eight", "nine"]
    return words[digit]

```

```python
def digit_words(digits: str) -> str:
    """
    Convert a string of digits into a space-separated string of their English word representations.
    
    Args:
        digits (str): A string containing digits.

    Returns:
        str: A space-separated string of the English word representations of the digits.
    """
    if not isinstance(digits, str):
        raise ValueError("Input must be a string of digits.")
    
    words = []
    for char in digits:
        if not char.isdigit():
            raise ValueError(f"Invalid character '{char}' found in input string.")
        words.append(digit_word(int(char)))
    
    return ' '.join(words)
```