Britney Spears' father thinks Kevin Federline is spending child support money on his 4 other kids

Kevin Federline has two kids with Britney Spears and four more with other women -- and if he's not spending his child support checks properly, that could pose some new problems for him in court.

Kevin is requesting an increase in child support cash from ex Britney, who currently sends him $20,000 a month to help with sons Jayden James and Sean. But Britney's father, Jamie Spears, isn't convinced that the $20,000 is exclusively going toward their shared kids.

Jamie is preparing to demand Kevin's 2017 tax return, according to TMZ, and the document will shed more light on how he's been spending those checks. It may prove that Kevin has been putting that annual $240,000 toward all six of his kids, and possibly other expenditures as well.

Sources say Jamie would be willing to delegate more money per month if a judge determines Jayden James and Sean need it, but he doesn't want the money to support anything other than the boys.

Kevin's other kids come from ex Shar Jackson and wife Victoria Prince. He and Britney were married from 2004 to 2006.