arrear interest for child support

Old cat and all friends,s.

I have a question to ask.The new discovery showed my ex didn't diclose his true income over the year. Hence the court ordered support amount is very small. Now we are changing the amount to the date whenever his salary was increased. Can the interest be charged to the difference? This is huge amount since the years and 10% rate in CA.

His attorey argued to pay difference only but interest since court didnt order it in the past. But the fact is that ex had never disclosed any proofs until court ordered the tax records.Even now, he only claimed he made this much, refused to provided pay stubs and claimed no tax filing in recent years.

I dont understand why he wont be punished if interest can't be charged and it caused the custody parent a lot legal fee to get it disclosed every time.

Thank you very much!