Since both numbers have to go into the court order now (one amount for two kids, then the other amount for just one child once the first child turns 18), you have to do two worksheets now.
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The goal is to make the future change in child support automatic, so that neither party has to drag themselves back into court to modify the child support amount after the each child "ages out", i.e. turns 18 (there are a handful of other reasons why child support would cease at a different time).
So basically, your first worksheet would establish child support from the day you filed your petition, until the oldest turns 18.
Then, the second worksheet would establish the child support amount from the day your oldest turns 18 until the youngest turns 18.
And child support ceases, automatically, when the youngest turns 18.
Again, there are a handful of other reasons why child support would cease at a different time.
Seek the advice of an experienced family law attorney to assist you in this matter.
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