Well, the simplest form of proof is his income taxes for the last 3 years. If line 150 on his personal income tax is accepted as 68k then he pays support based on that. Child support is generally based on gross personal income unless parties agree to their own amounts.
I am pretty much expecting the posters in this forum to jump all over on this. It comes across as greedy and unfair to go for more money than children really require. You'll have a hard time convincing anyone rational that children cost thousands of dollars per month in additional expenses that your ex should cover.
Even if your ex's taxes do say 140k, if in reality he is living on 68k (before personal income taxes) you will destroy the children's father's ability to offer a good home for them by taking all of his money. Kids should come first, and for that to be a consideration, BOTH parents need to involved as much as possible, and be able to have a good home for the kids during their time.
Just my opinion
Get a financial statement and his income tax returns if you can, if he refuses you could use court to file a motion for support.