Sadly the days of families living on one income are long gone, unless that family is surviving entirely on benefits, if they ever really existed at all. My mother always worked albeit part time around school hours and my sisters always worked, too.
And of course there is the rise in single parent families. If there is a choice between free childcare that allows the single parent to work or supporting the entire family on benefits, then maybe free childcare is the better option.
In an ideal, perfect, fairly tale world one parent would always be at home whilst the other worked and supported the family and both parents would be happy and content with their lot.
Grandparents can't be relied on to provide free childcare - they are at work and looking after their elderly parents in their spare time.