In general children want reassurance that they are loved and that there are boundaries set for them.
Make time to play with your little one, if (s)he cries a lot ask her what the matter is, and talk to the child about her feelings and that its OK to be sad/angry, but not OK to take it out on mummy.
Do a star chart so when he/she does so many good/helpful things in a row he/she gets a sticker and when the chart is full they get something they really want a childrens magazine a small toy (something cheap) or a favourite meal or something.
Don't take disrespect from your child, if he/she is being rude you tell them immediately 'no' that is not nice and put them in time out, or if he/she is really playing up, put him/her in a room where they cannot hurt themself and have no toys till they calm down. I used to do this with mine, they'd generally calm down after a few minutes and I'd go into the room to find the child singing/looking out of the window etc and eventually they would kiss me and tell me they were sorry they made me sad.
If you are currently in the middle of a divorce, do not make the childs father out to be a martyr, a wonderful human being, because that makes you a target for abuse and resentment from the child now and probably for the rest of the childs life. Because absence does make the heart fonder especially if you replace the real memories with fake rose tinted ones. If the child has witnessed abuse and talks about it, let him/her, then acknowledge that what he saw was naughty, and we don't do that because we love eachother and we love Allah and Allah is sad when we are naughty like that. I feel that pretending the abuse never happened especially if a child has witnessed it, is in itself abusive, your telling the child his/her memories are lies, and when children are young truth and lies are a huge deal to them.
Tell your child you love him/her often, you will always keep him safe inshallah, and make fun happy memories for one another, go to the park, go shopping together, get your child to memorise items your need to buy, allow him/her to help decide what to buy ie this apple or this banana etc.
You can do it inshallah, make time with you fun as well, structure your days, so your child knows what to expect, and just love him/her and always pray for him/her and get your child to make dua too it's really cute when little kids make dua and you say ameen.