The average adult human contains what, like 30 pounds of carbon? That's very little money according to the carbon offset prices I have seen.

You have to account for:

- all the meat and diary your child is going to consume in those 18 years

- all diapers that are going to be produced and disposed.

- all methane and CO2 you child produced on an a daily basis for those 18 years (exhaling, digesting/pooping/farting)

- concrete used for building this extra room for your child

...

just to name a few.