Polish original and English translation: http://simple.wikipedia.org/wiki/Wlaz%C5%82_kotek_na_p%C5%82otek

Lojban translation by ianek:

lo mlatu cu bitmu cpare sei
dei melbi selsanga be fa dei
nalclatei naltortei prane se'u
i doi mlatu ko sanga za'ure'u

You can download a recording of young people singing it at the Science Camp Serock 2011: mlatu.mp3