Let's look at the proposal in detail (yes, I know it is very unlikely to be implemented, but bear with me)
Firstly, GB wants to provide 10 hours free childcare a week for 2 year olds, but it will only be made available to the poorest 40% of families. IMO, that must work out as quite a small number of families. Doing the sums, that would work out at about £35 a week for 40 weeks a year or £1400 per year.
Secondly, to pay for it he proposes stopping salary sacrifice childcare vouchers (SSCVs). As these are available to most families using childcare, that policy must affect a much larger number of households and, in some of those households, both parents will be in the scheme. Also, don't forget, childcare can also mean breakfast and link clubs which are used by school age children. If a parent earns between £10K and £30K (as most do), and they sacrifice the full amount allowed, they will save £962 a year.
Therefore, by scrapping SSCVs, GB will save lots of £962 a year, and will pay out a much smaller number of £1400 a year.
So, what does he intend to do with the difference?