Hi and welcome to the board!
I would say that he just doesn't understand that you need help. If you could find a way to leave him alone with the kids for a few hours, he may start to realize that it requires all of your attention at those ages. My DH helps out a lot and does dishes, clothes, and mopping when I ask him to. Have you asked specifically what you would like him to do? Instead of saying "Could you help me with the house?" specifically ask him to do a certain task. Some men just don't believe housework is their job and won't do it.
I've learned that boys tend to not see what needs to be done unless we ask them to do it.
Hey there! I know exactly how you feel..my hubby gets at least 3 or 4 days off in a row and I feel sometimes he helps extremely too much and then the next week he barely helps at all. I have tried over and over to tell him even the little things mean the most, such as what you said: helping me give the kids baths or washing up a few dishes. It seems they think we don't do anything besides sit at home sometimes!!
I never realized how much hard work it is really to be at home until I started staying home recently after we moved. I so feel your pain. Email me or keep n touch, would love to have some new friends on here :)