Get as much information from the parents as you can, Draw up a list of questions to ask them and arrange a meeting before you start if you need to.
Find out exactly what the parents expect of you, will they have outings etc planned for you, will they provide money for days out/refreshments, travel expenses or will you be paying and then being reinbursed at a later date? (receipts usually required).
Do any of the children have additional needs or dietary requirements? allergies or phobias?
Find out their usual routine, general mealtimes. I would get as much information about the 2 year old's routine. Naptimes, mealtimes, food preferences, is he being potty trained? Sticking to his usual routine will help the toddler feel more secure in your care.
Also, ask the mum to teach you how to use the toddler's buggy! How to collapse it, use the brake. I remember starting a new job, driving to a lovely park and spending a disastrous half an hour trying to put the buggy up. 
For the first few days, I would concentrate on getting to know the children and doing simple short activities before planning a big day out.
Some of these points seem a bit picky, but they honestly will make your job easier. Good luck 