It's a maximum of 18 weeks unpaid until your child reaches 5 or in the case of adoption, until your child reaches 18, whichever is sooner. There are grounds around how you can take and request the parental leave. You should ask your employer for a copy of your parental leave policy.
I get a certain amount of unpaid parental leave days per year, seperate to my annual leave. I don't know if all companies offer this. My DCs are 9,13 and 14 but I've worked for the same company since dd was a baby.
If you have to stay off and look after an ill child,you are entitled to emergency leave for care of dependants. It's unpaid though, so lots of people use holiday instead. And it generally doesn't cover, say, looking after chicken pox for a fortnight.
this is the type of leave where it has to booked weeks or months in advance, I think? I never know what workplaces do if parents need to stay off work and look after an ill child. DH work has always taken it out of his holiday entitlement, so far.
Although 'emergency' leave for care of dependents doesn't stop at five. That can cover elderly parents, etc as well as children. But I assume you were thinking of something a little more long term and pre-arranged?