"The boys want a rabbit."
Translation:De jongens willen een konijn.
konijn is larger than a konijntje, but 'konijntje' could also refer to a very cute (little) rabbit as opposed to konijn which is just a rabbit. If the rabbit would appear on your plate and you enjoy to eat it, you could say: oh, lekker konijntje! (ooh, nice (little) rabbit). This is the same for if you eat a boiled egg: lekker eitje (nice egg!).
Other cases: I call my brother 'broertje', while he is close to 2 mtrs tall, but I could use this form for 2 reasons: - affection (he is my 'cute little' brother) - to indicate that is is younger or smaller (which he isn't in my case, so must be the first).
I always refer to my horses and dogs as 'paardjes' and 'hondjes', eventhough they are not small (and certainly not all the dogs are cute and little), but they are my furry friends (affectionate).