Have been following this one with interest...
my DH has two children with his first wife. We have one. We made the decision not to even contemplate changing the amount we pay to his Ex / the kids when our son was born.
For me, it's not really JUST about money.
In fact, we moved to a larger house so that both of his daughters could keep separate rooms. We felt it was important for his daughters to appreciate that nothing was changing for them.
I really disagree with people being able to go back and change amounts of money unless they HAVE to (and, IMHO, having more children is not a NECCESSARY change). I would be extremely angry if DH's ex wife tried to increase her money because my DH now earns more.. He's taken on more overtime to ensure that we can provide a similar lifestyle (not better not worse) for our son.
FWIW, My husband actually works abroad 5 days a week so I have a tiny idea what it's like to be a single parent... I have no support locally (we live here to be near DH's daughters), so I know what it's like trying to work around childcare, and what it's like having to look after a baby when you've had no sleep and feel like s**t...
But, IM(very humble)O, I don't think the CSA guidelines are enough. 10% of someone's salary is peanuts.