Such good advice here already!
A few things that have helped me with discipline are giving clear expectations in the form of a schedule, and being consistent in following through on what I have planned and what I have said.
We start school the same time each day, and we try to finish around the same time. This alleviates me descending upon their free time and saying, "Okay, now I want to start school".
Work is not negotiable - if it is in the guide, it gets done 100%. This alleviates dc trying to get out of work if they've been allowed to skip things.
If dc are having an attitude with me, I have them take 5 minutes in their room to change their attitude, and I change mine as well. This calms us both down, and then we pray about it together.
I have found dc's work is only as good as my diligence in correcting it. If I fall down on keeping up with my correcting, they fall down in doing their best. Likewise, if I fall down on my teaching, they fall down in their independent work. An encouraging, cheerful attitude on my part goes a long way to help my dc have an encouraging, cheerful attitude. This is the atmosphere of learning I want in my home, so I try even if I am not feeling the best or am tired because I was up during the night with one of my dc to be happy, to still show that I am very glad on even the most difficult days to be homeschooling them. Saying sincerely things like, "I'm so glad I get to be your teacher, isn't God good to make that something we get to do?" Or, "Today is going to be a beautiful day!" Or, "I can't wait to find out what's going to happen next in our Storytime book today!" Or, "You are one of my favorite people to be with - do you know that? I love you honey!" all go a long way to fill dc's emotional tanks and get them off on a good foot attitude-wise for school each day.
The more routine things can be the better. This reduces the need to "talk about everything". For example, it's helpful if each child knows where he is to work, when he is to work with me and when he is to work independently. It helps if each child has his own set of basic art supplies that are mobile too. It also helps if each child knows where to hand in his work. This alleviates all of these types of questions "I wanted to sit there!... He took my glue... Where's the scissors?... He left all his stuff all over the table... Why is Riley's book with my stuff?... etc.
Each of our dc has 2 plastic tubs, one for current homeschool books we are using and one for the books we'll need for the rest of the year. They are responsible for getting things out and putting them back within their tubs.
Having realistic expectations helps me homeschool more happily through the rough patches. Dc are not going to like everything we do. There are things that are not going to be their primary interests or their perfect learning style. As engaging and interesting as HOD makes school, it will still be work for both us as moms and our dc. Expecting dc to love everything is just not realistic. Work in life is not always fun, in fact, often it is not. Trying to cater to our dc's whims just turns out dc that are quite self-centered and expecting everything to be easy and interesting for them. I actually look at things that are less of an interest or more of a challenge for our dc as the best time to do some character training. Perseverance, diligence, choosing a cheerful attitude - all of these character traits can be honed in these situations. One other thing I've learned - as much as we love homeschooling - neither my dc nor I want it to take all day. I do not add, elongate, or elaborate on the HOD plans. We clip along and enjoy our time very much, but when we are done we feel a sense of joy and accomplishment and appreciate how homeschooling in this manner helps us have time to do other things with our day too.
As your dc are coming from a ps setting, I think I'd not allow comments such as "My other teacher didn't do it this way". It seems like that's a common comment from dc coming home to be homeschooled. A good response to this right away may be something like, "I'm your teacher now, and this is the way I do it. You would have had a different teacher this year in ps anyway, and he or she wouldn't have been just like last year's teacher either. Please do not make that comment again, or I will need to give you a consequence."
This establishes your authority right away.
HTH - but I think you're going to do just fine, and your dc are so blessed you are going to homeschool them! They may not thank us now, but I think they will someday.
In Christ,
Julie