You could be honest with the parents and say that you don't have experience of this age range, but are willing to learn. You could hold out for one of the "right age", but if you need the money, I would take on these three.
You'd obviously have to get some toys/activities for them - can you talk to them/their parents and find out what sort of things they like to do.
A trip around the ELC or Woolies checking out the "suggested age" stuff might help, and visit car boot sales.
I have minded girls of this age - they love typical "girly" stuff (jewellry making, dolls house, Barbies and so on) but also anything to do with felt pens, playdough, painting, sticking etc, which you would be getting for older toddlers anyway. They also like daft games (like Twister!), or bingo, dominoes etc.
My minded schoolchildren (age range 4 - 10 years) always have a snack/drink after school, and often just want to "chill out". In the summer, just being in the garden (ball games?) with water or sand play keeps them happy! Indoors, we do have a games console and will watch some TV (but not for the whole time).
It can be hard to juggle schoolchildren with babies, but I leave out certain activities that they can help themselves to if I am busy.