If he wants to go on and do science A Levels, then he really should (to the point of must) do triple science.
However, given that it seems he is
a) keen on the subject
b) not dropping anything else that would be more worthwhile for his future and is a realistic choice for him
I'd say go for it
You mention he can't wait to be a father - this may at least give him the knowledge that he needs to realise that having kids isn't as easy as he might imagine, and get him to delay fatherhood for a few years, until he has settled down!
However, it looks like GCSE Child Development has been phased out, and is no longer available www.gov.uk/government/uploads/system/uploads/attachment_data/file/479635/2015-05-14-reform-of-gcses-as-and-a-levels-in-2017-may-2015.pdf meaning that he would be studying for a qualification that is equivalent to GCSEs (so Level 2 on the National Qualifications Framework) but not an actual GCSE. This probably won't matter in the long run, but it's worthwhile making an informed decision.
I work for a university, and what I always tell kids picking their GCSEs is to pick ones that they
a) will enjoy
b) will do reasonably well in
c) will keep their options open for any A Level subjects that they want to take / are necessary for any future degree they want to take.
So long as he picks triple science, I can't see any problem with his GCSE options for a science based degree / career, and GCSE childcare does seem to tick all the boxes above.
Even Oxbridge does not operate some sort of GCSE blacklist where anyone with one of the subjects on their UCAS form is automatically rejected, regardless of what else is on there - that would be absurd. All they do is check that they have got the subjects that are required for any given course (and any requirements are always clearly stated on their website - there are no secret entry requirements that only special people in the know are told about). The only university that is sniffy about people not having a language GCSE is UCL - and for students who don't have one, they just have to do a language module once they're actually there, so it doesn't actually stop them getting in.
Oh - and if he does decide that he wants a career in childcare, then there are some very good and well paying jobs out there for men, as they are so rare in the industry! For instance, there was a male student at Norland College, and their graduates can earn £50k per year nannying... www.telegraph.co.uk/education/educationnews/9526118/Meet-the-Norland-Manny-teenager-is-first-undergraduate-at-worlds-most-famous-nanny-academy.html