It's the photo NO ONE ever thought would be taken ... Charlie Sheen and his ex-wife Brooke Mueller TOGETHER with their kids this weekend ... smiling, like one big happy family.
The meeting took place at Charlie's Hollywood Hills mansion Sunday -- while Brooke was on a break from her new rehab program.
TMZ spoke with Charlie ... who told us the twins, Bob and Max, had a great time -- playing around and scarfing down handfuls of candy while BOTH of their parents watched.
Charlie says, "Harmony is our goal. Sunday was a wonderful start."
Charlie and Brooke are working hard to mend their relationship so they can co-parent their kids. We're also told ... when Brooke completes her treatment, Charlie wants to move her to a home in his neighborhood, so they can spend more time together ... as a family.