Seven wins from their last eight games at Oakwell had helped see the South Yorkshire side move out of the bottom three, but defeat against mid-table Charlton leaves them inside the drop zone, albeit one point behind Blackburn, who are 21st.
Bradley Pritchard gave Charlton a fourth-minute lead before Johnnie Jackson doubled their advantage.
Yann Kermorgant headed a third after 48 minutes before Stephen Dawson was shown a straight red for a lunging tackle on the Frenchman.
Callum Harriott made it four before substitute Salim Kerkar added a fifth as Charlton claimed their ninth away victory of the season to move onto 57 points, six off the play-off places.
A horrible day for the home side was capped by a second sending off when Tom Kennedy brought down Ricardo Fuller when last man. And it was Fuller who made it six in stoppage time.
Just four minutes had been played when a Chris Solly cross found an unmarked Kermorgant whose shot could only be parried by Luke Steele into the path of Pritchard who tapped home from close range.
The home side had plenty of the ball in the middle of the park but lacked the imagination in the final third to open up Charlton.
It suited the Addicks who were dangerous on the counter with wingmen Pritchard and Harriott often options for Fuller.
Only the top three have won more on the road than Charlton, and they doubled their lead after 19 minutes with a well-crafted goal.
The Addicks exchanged passes on the edge of the Barnsley box before Jackson feigned causing two home defenders to slide in.
The captain took the ball forward before firing a powerful effort between Steele's legs.
The goal sapped the life out of the home side and Jackson almost had a second when his curling cross had to be tipped over the bar by a scrambling Steele.
With his side struggling to get into the game Flitcroft brought on striker Jason Scotland for Cywka after half an hour to go from a 4-4-2 to a 4-3-1-2.
The forward made an immediate impact waltzing round Jackson before playing in Jacob Mellis whose shot from the edge of the area was cleared of the line by Rhoys Wiggins.
The home side made a far more positive start to the second period and clever play again from Scotland played in Chris Dagnall, who, off balance, fired wide of Ben Hamer's near post.
However any hope they had of getting back into the game was ended moments later as a brilliant Wiggins cross evaded everyone but Kermorgant at the back post who headed home unchallenged.
Harewood came on for O'Grady after 57 minutes but moments later it was four and again it was down to a Steele error.
Harriott's 20-yard effort had power but it should have been a comfortable save for the goalkeeper. However he let the ball go in under his body and the match was well and truly Charlton's.
It got worse for Barnsley as they soon found themselves down to 10 men after Dawson went in strongly on Kermorgant and although he seemingly won the ball, referee Geoff Eltringham took little time to brandish a red card.
There was still half an hour to go and Barnsley were in danger of receiving a real hammering.
With 10 minutes left it was five when good hold-up play by Fuller brought in substitute Kerkar who thundered an effort past Steele.
There was still time for another Barnsley sending off when a desperate Kennedy fouled Fuller when 25 yards out when the Jamaican had just the keeper to beat.
Fuller curled in a sixth in stoppage time after Barnsley once again failed to clear their lines.
- Sports & Recreation
- Ricardo Fuller