I have an 18 month old daughter who does wonderfully with older children but I've recently started watching another little girl similar in age during the day and all hell breaks loose. My daughter kicks and screams and cries and tries to push and hit the other little girl. I don't know what to do. It's been three weeks and it hasn't gotten any better. Is there anything I can do to help them get along?!
Really I think the best thing to do is to tail them when they're going through a stage like this and respond gently but consistenly. "Anna can use that toy too. These toys are for sharing." Catch her hands as she tries to hit and say, "We don't hit. No hitting." But you do have to be right on top of them for a period of time and just catch them as they do it. If she's only having trouble with one child, then you might be able to only tail her while she's with that one child. Hopefully it'll be just that one! They can go into and out of this during toddlerhood, so she might get past it with that child and then get a bit aggressive with someone else at some point. But talk to her about it and catch her every single time, and she'll get the message. Talk briefly, as they aren't taking in much at that age, so "no hitting" or something equally short, and beware of "don't", even though I used it earlier in an example. Some toddlers don't really understand contractions as well as other language so it can sound a bit like "blah blah hit." Also, it puts the image of hitting in their head when you talk about it in a negative like that, so you could also try, instead of "no hitting", something like, "Hands to ourselves" or "gentle touch" or something phrased in a positive. Maybe try a few things and see what she seems to understand and respond to the best.
I have twins, so we went through a few squabbles at that age. I think the most effective thing to do is to tell your dd what she CAN do, rather than what she can't. "We talk nicely. "We are sharing-- it's her turn now and when she's finished it will be your turn." "Gentle touches." "Use your words-- tell her she can play with it when you're done." "Since you're not using the marker, it's her turn."
I have to admit that I closely supervised my boys, so they never hit, but there was some toy grabbing. But I think at that age, supervision with more than 1 little one involves getting down on the floor and being right next to them so they can have constant feedback.
The good news is, this is a phase and the work you put in now will pay off sooner rather than later. My boys never fight and I'm always amazed when otherwise nice kids tell me about knock-down fights with their sibs. That would NEVER happen in our house; situations just never escalate.