Hello! Were the children like this before their father was deployed? If they were, then the discipline problems are there because they were never set any boundaries in the beginning. If it's happened since he left, then the children are not only probably acting out because their dad isn't around, but moreso because their "daddy's not around" and they know that they can get away with it with mommy. See the difference?
The kids need to be able to predict what the consequences are of their actions. That's the only way they'll learn what they can and cannot get away with. One thing I did with my daughter when she was 3, was make a "discipline chart". In my opinion, I believe that this is the age where they really start to see how much they can get away with. There were about 5 different things that she was doing that she was getting in trouble for. They were anything from lying, to hitting, or not listening, etc. I didn't feel that if I did the same punishment each time, it would have a consistent lesson behind it. So, I sat down with her, and together we decided what punishment each action had. They were things like "time out, giving mommy some money out of her piggy bank, spanking, etc." This worked really well, mainly because it worked really well for me. I found that I was no longer getting all mad during the day, because instead of getting worked up, and then trying to figure out how to handle her behavior, it was all written out for me. I would just take her hand, walk over to the chart, and read what her punishment was. I strongly recommend this for mothers who are having trouble staying calm during the day because they find that they're kids are "always getting in trouble". Back to your daughter, if one bites the other and sometimes doesn't get in trouble with it, then they will always take the chance that they "might get away with it this time".
If the kids aren't listening to her, I would do something like take toys away, turn off the tv...something that they will have to earn back. If the behavior is physical, I would get down to his level and look him in the eyes and tell him that biting, kicking, or whatever, is not acceptable. She can try a time out, but sometimes that doesn't work. Something like hitting or kicking, really needs a harsher punishment. I would figure out what toy he has that is very important to him, and I would take it away. I would get down on my knees, look him in the eyes and say "honey, I know that you're sad that I took mr. snuggles ( you catch the drift) away, I know how much he means to you. I will be happy to give him back to you when you've earned him back. To do that, there will be no more hitting or kicking for the rest of the day." If he does it again, take the next favorite toy away.
My little boy who's 3 could care less about a time out, so she may find that a time out doesn't work. With my son, I take him into his room and sit him on his bed and tell him he's not allowed to get down until I come and get him. If he gets down, I don't talk to him, put him back on the bed, and walk away. I find that this works well when he's whining about something or isn't sharing his toys or something, something that isn't as necessary for as harsh of a punishment.
Once she establishes that she is the one in control, she will find that the kids will start to behave. Once the kids realize that they can't push mom around, they will start to listen. Remind her that no matter how hard it is to constantly be motivated to enforce the rules of the house, it's absolutely imperative that it's done. It's not only for her sake, but for the sake of the children. Children need to understand the concepts of consequences. They need structure, and they need to feel like they're good children. It's kinda hard for that to happen when they're acting up all the time and are in trouble for half the day. If you're constantly at them because they're misbehaving, it kinda "brainwashes" them, for lack of better word this early in the morning, into thinking that they really are a bad kid, and then they're actions will follow what they believe they should be doing. Also, in the meantime, until this behavior is under control, please tell her to always always always take notice of the times when they are being good. On a day when I feel that my kids are being really bad, I always go to each of them when they're being good, even if it's only for 5 minutes that they've been good, and I pick them up, give them hugs and kisses, and tell them I love them.