It depends which explorer. Overall they were heroes, but there were certainly explorers who were brutal and cruel with their treatment of the peoples they "discovered".
In early years, kids should learn about the good they did, to keep it simple for them to understand, and to help increase their imagination, and then as they grow up, they should teach the bad things they also did. 50/50
It obviously depends on the explorer, but I think they should be viewed as neutral so the students can decide for themselves.