frustrated and concerned

Hi! I'm new to the forum but have been providing in home daycare for many years. I am looking for advice on a situation that has come up. I care for all age children, my oldest is 4 and my youngest is 7 months old. I currently care for 5 children and love what I do. I have 2 children of my own, both over the age of 12. My situation concerns the youngest child I care for. The family is new to the daycare scene, before they came to me for childcare the baby was being watched by grandma. Grandma still watches the child on Thursday and Friday. The first week the child was in my care the mother made a comment when she returned the following Monday morning to drop the child off. She was concerned about a small bruise on her forearm that she noticed on Saturday. She didn't know if maybe one of the other children at the daycare had tried to take a toy from her or something. She did acknowledge that it could have happened at grandmas. She tried to show me the bruise but couldn't find it. A few days later she commented on a small bite on her arm, close to her hand. Again, she couldn't find the bite the next day but at this point I'm beginning to get uncomfortable. I want my parents to feel comfortable with leaving their children in my care and I understand how difficult it is to leave your child with a stranger. As I said, I have two children of my own and understand how difficult it is to trust someone with your child. The child has not been in my care for very long, less than a month. It's becoming a stressful situation. The third week she was here, the baby had a long but very thin scratch on her leg near her diaper. Mom was able to show me that, it looked like a thin scratch from a hang nail. I saw it the day before and meant to say something to the mother but unfortunately forgot. I do have a dog and she thought it maybe from the dog. Which she admitted didn't make much since because the baby had a sleeper on the day before. Today, the mother was dropping of the child and she had her on the floor in front of her, between her and an activity center. The 4 year old that I watch was trying to walk between the small child and the activity center. I told the 4 year old that she needed to walk around the baby and she stopped and did what I asked. The mother picked the baby up, walked over to me and said "I think she (the 4 year old) has a problem with my baby". Of course, I was shocked and asked her what she meant. She said she thinks that because she's the oldest child here that she may be jealous of the baby. She said she didn't have any proof but just felt that she was doing "something" to the baby. At this point I'm frustrated and concerned. Can anyone help?