I'm dutch, but have been living in Germany for 6 years now.
sliep uit
sliep is pronounced like sleep no problems there
uit (meaning "out") has the uniquely dutch combination of vowels ui, which is pronounce something between oy and ow, very difficult for anyone not born and raised with it.
anyhow, to the meaning of the expression. You should know that it is accompanied by a hand sign in which the index finger of one hand is repeatedly moved down the index of the other, the latter pointed towards the "victim." This is a simulation of sharpening a knife. And that's where it gets interesting. The verb [sliepen] means to sharpen (of knifes, scissors, etc), and the verb [uitsliepen] means to mock. The latter verb is archaic, though (the first one as well in the meantime, when was the last time you saw a knife-sharpener in the streets?). Anyhow, mocking one among them, kids make this sharpening sign and sing the song to accompany it. It means
Sliep uit, sliep uit!
mock, mock!
Alle kinderen lachen je uit!
all children laugh at you!
or iedereen = everyone laughs at you.
the nênnuh nênnuh nêê nêê is close to your nyah nyah nyah