Trailing 3-1 entering the fifth, Curacao had runners on second and third when Koeiman hit a fly ball to left with one out. Carter Kada-Wong fell backward to make the catch, but both runners had enough time to score.
Jansen then homered to give Curacao the lead for good.
Reliever Rallison Bentura pitched 2 2-3 scoreless innings to get the win.
"I had the attitude that nobody could hit me," the 12-year-old right-hander said through an interpreter.
Cole Dalla-Zanna was the hard-luck loser for Canada, striking out nine.
"Our pitcher threw unbelievable. I'm not unhappy with the effort, that's for sure," Canadian manager Vito Bordignon said.
Canada took a 3-1 lead in the fourth when Noah Hanson-Stafford came off the bench to hit a two-run homer.
Canada was eliminated but Bordignon wants his players to be proud of their showing this year, which included a 13-9 win over formidable Mexico.
Next up for Canada: a little sightseeing in Pennsylvania.
"I just told them they had a great tournament and to keep their heads up," Bordignon said. "They represented Canada well."