Charlie Sheen's Twins -- New Investigation Over Injury to Bob

The L.A. County Dept. of Children and Family Services has opened a fresh investigation into allegations that someone hurt Bob, one of Charlie Sheen and Brooke Mueller's twins ... and we've learned Brooke is pointing the finger at Denise Richards.

Sources familiar with the situation tell TMZ ... Brooke reported the incident over the weekend, just after Charlie appeared on TMZ Live and railed on her. We've learned Brooke told DCFS ... there was a cut on Bob's back that was deep enough to scab over, and she said it happened on Denise's watch.

As you know ... Denise was the guardian of the twins while Brooke tried to beat her latest bout of drug abuse. Denise threw in the towel yesterday, saying the kids were too violent, and now Brooke's brother Scott is the guardian.

We're told social workers went to Bob's school and determined the cut had been there for more than a week, which means it could have happened while Bob was under Brooke's care. DCFS then interviewed Denise, who said the cut had been there for more than a week.

Our DCFS sources say the timing of Brooke's complaint is suspect because the accusation came on the heels of Charlie attacking her on TMZ Live.