It isn't just about the money. It's about the position you'll be in a few years time. There's very few careers on your salary that allow you to progress up the ladder working pt. Ft you might not be better off much now, but in a few years promotion could mean you are. Not to mention when he's older and you don't have much choice but to work ft.
The only way I'd say that pt is the better option long term is if you intend to use the free time for qualifications.
And on that income even £600 a year could make a significant difference.
I have also tried out a £35k job (assuming I could get back to similar level) that requires a commute and it wouldn't leave me much better off but is a logistical nightmare. (Difft school drop offs, difft childminders).
I know long term it's better but DC are very young, one not yet at school, and quite a bit of weekend time with their dad so I'd barely see them. So need to weigh up immediate needs.
Would I be under pressure from dwp immediately to find full time hours? I've no idea what is required.
The only way I'd say that pt is the better option long term is if you intend to use the free time for qualifications. I know that sounds easy but actually between 9 and 2.30 I'd be working 4 hours each day so it's not lots of free time, it's time with DC.