I always read these sort of threads with interest - one of the main things that impresses me is that so many people manage well on salaries at the lower end of the scale....
I think it's very hard to make comparisons though, as costs vary depending on where you live. For example, we have what is a fairly low mortgage for this area (less than £50k left I think) which costs about £500 per month to pay off - this might be considered high in other areas. Our council tax is £150 a month. Also, everyone has their own ideas on what they think is important to them and so what they will spend their money on- it might be having a night out every week or new clothes regularly or a good holiday every year. In our case, our families live in Ireland so a lot of our spare cash goes on visits there. Even with cheap flights and staying with family, a long weekend costs around £300 when you take into account car hire and airport parking.
When I worked full time and before dh got made redundant and changed to a lower paid job, our combined gross income was about £100k. I look back now and think, "What did we spend it on?" and "Why didn't we buy a holiday home somewhere?" but in reality I don't remember feeling like we had loads of spare cash. The difference was that we could do things like book a holiday and pay for it, without saving up first or worrying about how we could afford it.
Childcare costs a lot where we live - a fulltime nursery place is £800 a month and I doubt if a childminder is much cheaper (they weren't whan I looked into it a few years ago). If you have two pre-schoolers you have to earn a pretty good salary to make staying at work worth your while financially.
The other thing that affects things is how old you children are - if they do lots of after-school activities it costs a fair bit every term. So you might be doing ok when they are toddlers, but it can change when they start school.