i prefer
1. rhymes,
2. short and simple words,
3.rhythm (such as in possible alternate unstressed and stressed syllables)
like the one i wrote yesterday for my 徒弟：
a little birdie flies in sky
sometimes low and sometimes high
when low, he picks at peach and pear
when high, he nibbles tasteless air
please notice alliteration in line 3
by the way, i am not sure if this is a 兒歌，because i seldom read a real 兒歌 by a native speaker, ha
'我感覺topsy.turvy語言功底也很好，他的遣詞造句精煉，邏輯思維也很強。', true, that's why we should 對他更要嚴要求， haha
