Have I made a mistake taken on this childminding job when I have 2 kids of my own?

My husband has gone back to college. To try and get some extra money I decided to apply for a position for minding a little girl who is almost 5 just starting school. I have two girls, aged 6 and 2. This is for 3 days a week 1-5. I have only had her 3 days and I am starting to regret it. She seems to be quite a handful. I didnt realise how good my two were. My girls play happily together every day. Whether its playing barbies, dolls house or watching a movie. I can cook the dinner etc and I know I can trust them in the other room. This girl seems to be a different story. I can let her out of my sight. Its not that she is "supernanny ". She seems to be quite heavy handed with my 2 year old. Pulling out of her, shoving a bit, grabbing toys. Basically just upsetting her. She tried to climb into my girls dolly high chair and after tried to squirt baby shampoo over my carpet. Her first time allowed upstairs and after 10 minutes I had to take her down. On day one I started a diary for all the girls with good behaviour stickers. 10 stickers getting a treat. Listing what was good behaviour and what would warrant the step. I fill in every day what she eats and drinks and how good she is. Am I just being petty here? Her mother has told me in the last childminding placement her daughter got into trouble for running across roads, physically fighting and last week she bit her mother so hard it left a mark for 3 days. This was after I had started to mind her. I really dont want my girls to start behaving like this girl. Am I over reacting? Can any childminders give me some advice? I would really appreciate it. Thanks in advance.