I am no expert in this but I would have thought the answer will depend on whether you are their employee as yet or not until Jan 2008. If you have already signed on the dotted line, etc then I believe they have to consider your proposition although they don't have to actually do it. My advice (for what its worth!) is to firstly have a very firm idea of your childcare arrangements will be and then speak to your boss and explain your predicament. Bit difficult to suggest when to have this conversation. If you do so now at least you have enough time to find another job if need be and if too close to the date it may be risky if your boss doesn't agree. Tricky one.
Re: whether he thinks you are taking the piss. I guess this depends on what he's like, ie. How sympathetic is he to people with kids, etc.
TBH it's just one of those things so he should be able to see that.