'아이를 키운다는 건 참 힘든 일이에요' '아이를 키운 건 참 힘든 일이에요'?
(I changed 힘들 into 힘든 because it is more often used, meaning 'tiring'. It is also understandable to use '힘들' which means 'to be tiring' but I haven't heard this before.)
키운다는 means more general term of bringing up (a child).
'아이를 키운다는 건 참 힘든 일이에요' means: To bring up a child is a very tiring thing.
키운 is in past tense. So '아이를 키운 건 참 힘든 일이에요.' would literally mean: To have brought up a child is a very tiring thing.
The second sentence seems quite strange though... So when speaking about the past, we could say '아이를 키우면서 참 힘들었어요.' While bringing up the child, it was very tough(tiring).