Child tax credits only apply if you have a child, obviously.
Both parents, if a couple, need to be each working over a certain number of hours, which have been increased recently, and you can get varying amounts up to quite a high combined wage, which they keep changing it seems.
If you pay childcare costs, you can get paid a certain amount towards that, up to 70%, but that depends on how much you pay and how much you earn.
They say that you should let them know any changes in income and childcare costs as you go along, and they will adjust payments.
They used to pay extra if you had a baby under 1yo, but they have scrapped that recently too.
For more info just google child tax credits. There is a tax credit calculator that can give an indication of how much you may be entitled to.
Although I am in quite a well paid job, DH was a student for 6 years and now earns minimum wage (gosh that was worth it!) and is employed on a sessional basis. We could not afford childcare without the tax credits, as we were paying out around 1K a month. No idea how we will be able to afford this one too. I will probably have to drop to part time. I do get frustrated that most of my clients are on benefits, including DLA, and have a much higher disposable income than me, and never need to worry if they can afford another child. But I guess I do not have to worry about that child possibly being taken off me.