struggling first time mom

So me and my husband have full custody of his two children from his previous marriage. They are 4 and 5. On top of that I just gave birth and my son is almost two weeks old. My 4 and 5 year old don't understand why we can't go to the park, library, etc. I feel terrible because before the baby I would take them to all kinds of places. Now with the baby I feel like I have no time for anything. I don't think I've actually played with them since he has been born because the baby takes up most of my time. I send them upstairs to play and watch cartoons for most of the day.  My husband just went back to work and he works from 9am-6pm and takes night classes to finish his degree, so he isn't really here to help. What can I do? I feel like a terrible mom to them.