You are not alone! My boys just started public school this year (one in kindergarten, one in 1st grade). A new school, new lunch schedule, new morning drop off routine, riding the bus in the afternoon and both with homework! I was more nervous than them I think on the first day. Especially when I think my last experience with elementary school was when I was in it.
If I may make a few suggestions... first, have your husband take her bag and keep it in the front seat when he picks her up. That way she can't pull everything out on the way home. Second, is your son napping when she gets home? I don't have a baby, but I do make sure the first thing the boys do once they are home (shoes off and a few minutes to relax of course) is do their homework. That way I'm not distracted and they are still fresh in the school mindset. If your son is asleep when she gets home that might be the perfect time to attack the homework. You'll be set for it and then you'll know it's done and it won't weigh on you all afternoon. If you can't do it right after school, have your hubby entertain your son if he can while you do homework with your daughter. Third, is it possible for you to just put the homework back in her bag? If it isn't due until Friday then perhaps it can just stay in her bag until then, in a folder or gallon plastic baggie. Finally, if you haven't yet, speak with her teacher. Let her know you're feeling overwhelmed, that you don't always have all the supplies needed and that you don't always know what to do with the activities that come home. I ran into the latter problem last year when my oldest was in K and boy was it frustrating. Especially b/c by the time he got home he usually couldn't remember what he was supposed to do. In any case, if her teacher is a good one, she'll be more than happy to help you. After all, we're a team, us and our kids' teachers.
To wrap up this long reply, you are not alone. Being a mom is the biggest, bestest, hardest, most rewarding and most important job any woman can do. We want to do it right and do right by our kids. Just remember you aren't in it alone and that there's always someone to ask for help if you need it. Good luck and hang in there. You'll get into the swing of it before you know it. :-D