My kids are maybe too young to understand right now, but my neices are in my care often and they are old enough (my guinea pigs). I explain that there are things in life that nobody likes to do, but we have to. I wish I could stay at home and play with them, but then who will pay for the house? Homework may not be enjoyable, but if they don't earn good grades, they will not have things they would like. Everybody has to do something they don't like. It's Life!
i would try to explain to them that if they are good at school that they may end up having a job that they love and if you are lucky and love what you do then you will be a success. if they are sad because you are not home with them, when you are make it fun. make doing their homework with them fun. i know you are tired, but kids are kids and they wont understand until it is their turn so i would try not to explain and just try to get them to be happy. responsibility comes with rewards some times. so if they have no choice but to do something that they dont like, you can always reward them afterwards with something they do like. life is short, let kids be kids and let us adults do all the worrying.