As a father of 2 young children who works full time, I'd say start off with 30 credit points first.
I've just completed a HNC, and that required a lot of work at home, and I struggled.
I'm not one that buys into this 16 hours per week for set number of points, or 30 hoursp er week for these set of points. It all depends on other factors. IE: how much overtime you may have to work. In November/December I'll work all hours under the sun to provide a decent xmas, so those 2 months are bad months for any spare time. My partner would look after the kids, and I'd be home on a Friday at 9pm every single Friday, missing a long afternoon/night in studying.
Then there's sickness that kids are prone to, school nights like discos and so on, a 2 week holiday abroad perhaps in the summer? All kind of things come into it now and again where 16 weeks every single week just isn't possible with a young family, then there's the days where you just need to chill for a night or 2 before getting back into it
It's up to you though, and how well you are able to carry this out, and if you have a lot of family/friends that may be able to give you a nights peace every now and then (my partner and I don't have this luxury) and how good sleepers your kids are. Mine are 2 and 6, and like to take turn about in getting up, or any excuse to stay awake, which means that although we put them to bed at 7pm, that doesn't always give me the rest of the night, because it can be 9, half 9 before we get sat down, and at that time, it's sometimes too late to function..