Your 18-55 on a Nikon DX body (D5000) will give similar field of view to a 27 – 83mm lens on a film camera. So depending on what lens you used before, this should allow you to assess suitability. You might need a longer lens. For the range given I suspect you’ll be working mostly at the long end of the 18-55
At 10 – 20 m range you won’t be able to use fill flash so preferably shoot with the sun behind you and early or late to get good colour. If possible, try to lessen the complexity of the background behind the animals because depth of field with a f4 to f5.6 lens is going to be deeper than the animal at your stated range. Nearby background items are likely to be in focus and therefore distracting.