Soulja Boy Exploits Chris Brown's Daughter in Feud

Well, it just got real ... Chris Brown's daughter, Royalty, is now smack in the middle of his beef with Soulja Boy -- and Soulja's the one who put her there.

SB just posted a pic of 2-year-old Royalty with Floyd Mayweather and TheMoneyTeam tags on it -- making it seem like Chris' little girl wants to see Daddy get his ass whooped. To say, "low blow" ... would be an understatement.

Here's the thing -- Soulja didn't put the Floyd tags on the pic. Chris' baby mama, Nia Guzman, did ... Soulja just reposted it. So, does Nia want Chris to get his ass kicked?

Probably. But we spoke to her, and she says her Royalty pic had nothing to do with Chris and Soulja's celeb boxing match. Nia says she and the kid have been hanging out with Floyd recently ... and "Floyd is investing in Royalty's future."

Hmmm ...

As you can imagine, Chris is PISSED off and making it very clear to Soulja ... leave my daughter out of this -- but with much more colorful language.