If he has 0 prospects where he is, and would have prospects in this new job, then your advice is incredibly short-sighted.
OP, if you genuinely think this job will lead to a better life i a few years then you should take it. Think of the long game.
Accepting all the negatives on the first job the OP has been offered where the only positive is "prospects" is extremely short sighted...
Prospects are worth nothing if they don't come to fruition and if you take a minute to look through some of the other threads in here it happens all to often that XY and Z are promised and never arrive.
OP while I respect what you are saying, I am advising that this seems like a very poor choice for you to be making.
While you say you will take a £700 per year drop but will save money on child care you still have the £700 loss as you will still be saving the child care money.
The saving in child care is effectively a net rise of £1680 (or equivalent of £2150 gross) staying where you are or moving so to mention it is a bit of a moot point (based on the information you have provided so far).
Losing 8 days holiday is big. You are losing over 20% of your current holiday. If your current work place allow you to sell them back holiday allowance then you can do this and reduce holiday to the same as the new place and be even better off. If you cant, you have 8 days more holiday that you can use more freely then fixed to bank holidays.
A 25 minute drive I will deduce a 15 mile commute (30mph average factors in town and b roads, not motorway, if this commute is motor way this cost just goes up). 30 miles per day 5 days per week giving a fair mpg of 35 works out to £90 in petrol per month, another cost of £1080 from your net, or effectively £1380 gross raise.
Overall you are looking at a new job at the bottom of the food chain with "prospects" for a net cost of £1780 and this doesn't factor in the value of the 8 days extra holiday (at £20k per year this is £600).
To get you back to the position you are in now you would need a gross raise of £2280, so even with prospects this might take a while.
I feel my feedback is not short sighted in the least here as I have factored in all the apparent negatives, while the positives are intangible.