My first reaction would be not to take the job! Can the one year old even speak his or her own first language yet??
If you only have the option to split them into two groups, I'd put the one and two year old together and the three year old with the five and six year olds. You can drill with flashcards etc from three, but not with 2 year olds. Three groups would be better, with the two year old either with the 1 year old or the 2 year old, depending on how mentally developed they are etc.
The only thing you can do if they are all together or in just two groups is take in a whole load of games, toys and songs and use English with them as you play. They won't learn to produce very much, but they should be able to follow instructions and sing along.