'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).

아이를 키운다는 건 참 힘든 일이에요 아이를 키우는 건 참 힘든 일이에요 these two sentences are similar but slightly different. the difference between them is revealed if we place '저는' at the start. 저는 아이를 키운다는 건 참 힘든 일이에요 this says 'it's hard for me to raise a child'. but if we do the same for the other sentence, it won't make sense. 저는 아이를 키우는 건 참 힘든 일이에요 this doesn't make sense because the topic particle 는 is marking both 저 and 것 as the topic of the sentence. this second sentence can make sense if we change it to either of the following: 저는 아이를 키우는 것입니다 아이를 키우는건 참 힘든 일이에요 which is the second sentence you originally wrote and this means 'it is difficult to raise a baby'. with "아이를 키운다는 건 참 힘든 일이에요" the 저는 is omitted but its presence is implied. so that first sentence you wrote means 'it is difficult *for me* to raise a baby'.