1. You're testifying and arguing law in the same document (¶1 and 3 law; ¶ 2, facts). This is bad form for a lawyer, but you're not a lawyer, and your opponent is now in pro per, so maybe it won't matter to the judge.
2. A "clear intent defraud" is not the same as actually committing fraud. The question is: (a) What did petitioner misrepresent to the court?
(b) Did that misrepresentation produce a materially more favorable child support
award (at least $50 per month)?
(c) Did respondent rely upon that misrepresentation in stipulating to the previous support order?
If yes, then the court will set aside the prior order.
3. You need to set forth facts (FL-300 Section 10) which show exactly how and when petitioner made the misrepresentation. Example:
1. On ??/??/????, Petitioner wrote in a declaration to the court, "I will be filing my 2012 taxes as married-separately with one dependent." See Exhibit #1, attached.
2. The court order of child support, entered ??/??/????, is based upon an xSpouse child support software calculation wherein Petitioner would be filing taxes as married-separately with one dependent (see Exhibit #2 attached)
3. On ??/??/????, Petitioner provided a copy of his 2-12 Personal 1040 tax return to respondent, in which he filed his taxes as married-jointly with four dependents. See Exhibit #3, attached.
4. Using the xSpouse child support software, I have calculated child support based upon the correct application of Petitioner's misrepresented filing status. The difference in payments is $X. See Exhibit #4, attached.
5. Had I known that Petitioner's actual filing status on his tax return would be materially different than the filing status purported in his declarations to the court, I would not have stipulated to the prior child support order.
6. [I'm taking a liberty here, but for simplicity sake, and because you and your ex are representing yourselves, you can probably get away with writing the following as your entire points and authorities in support of your motion to set aside] Points and Authorities: In re Marriage of Zimmerman (2010) 183 Cal.App.4th 900 (support order may be set aside within six months after within six months after date complaining party discovered or reasonably should have discovered the fraud or perjury. FC 3691(a)-(b)); In re Marriage of Economou (1990) 224 Cal.App.3d 1466 (Trial court did not err in making spousal and child support retroactive after setting aside fraudulently induced stipulation where husband was put on notice that retroactive child and spousal support
would be considered and was offered opportunity to present his views on the matter.)
That's all you need.
Ordinarily, statements made during negotiations by either party or counsel are inadmissible over objection. Evid. Code 1152(a). If you use evidence outside of the court record, you may get shut down -- either because your ex knows this bit of evidence law, or because the judge feels like playing "Judge Judy" and excludes your evidence without objection by your ex. I have no control over this possibility. If you want to avoid the risk, find evidence in a source other than during a negotiation.
Hope this helps.