When you start the children might have difficulty adjusting to you, its will take a bit of work by you to earn their trust and co-operation and the first few weeks are vital as this will set the tone for the duration of your stay. Therefore you want to try your best to get off to a good start. One of the best things you can do is be flexible and try to adapt as well as possible to establish a new routine. The children will feel more secure if you establish a good routine with them as this breeds familiarity with them and builds up daily patterns with them.
If the children are being generally difficult it could be their way of testing you out children are very clever, but try to avoid confrontations as much as you can by making it fun for them to get on with you. They might be nervous of you because you are new to them and mat react every time you go near them, remember they are not doing this deliberately and it will get better as time goes by so be tolerant and it will pass, it will take a bit of patience but the rewards will be great as soon as you have them as your new friends.