the childcare element of the working tax credit means I already subsidize (through taxes) other kids nursery fees while paying all of my own myself. (With two kids, that'll be 75% of my take home salary - it's a significant financial hit).
And while I admire some of the parents who are struggling to work with low incomes (offer lifts in my car since they don't have one), there's other parents that I know are getting CTC who drop their kids off and then call in sick and don't go into work because they're tired/bored/busy. So no, I would not chip in to a extra fund.
(Of course, specific family circumstances presented to me might make me feel differently, but that would be on a case-by-case basis and I would not subscribe to the idea in general).