A fragile peace has broken out between famed foes Brooke Mueller and Charlie Sheen — with the serial rehabbed socialite allowing her ex-husband to visit their twin sons — and we've learned the secret reason behind this surprising development.

Mueller, 36, is “desperate” to have $55,000 in child support re-instated, a source revealed, after it was slashed in half when she lost custody of the boys in the wake of being placed on an involuntary psychiatric hold after a drug overdose in May.

Said one insider: "There is only one reason why Brooke allowed the boys (Bob and Max, four) to go over to Charlie's house last week. She is desperate have her full child support payment. Brooke is a smart woman and knows if she is in Charlie's good graces, he is more likely to give her the money.

"It has been very hard for Brooke to live on $20,000-a-month because she has to pay for her sober coaches and ongoing expenses related to her recovery."

Breaking Celebrity News broke the story: Mueller allowed the twin sons to spend two hours with their dad and his [