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 儿歌，because i seldom read a real 儿歌 by a native speaker, ha
'我感觉topsy.turvy语言功底也很好，他的遣词造句精炼，逻辑思维也很强。', true, that's why we should 对他更要严要求， haha
