1.when 当心&小心 is used as a verb,they are both like "beware/be careful in(of)/watch out"in English
They can be followed by a noun,a pronoun, a sentence or used alone at the beginning of a sentence.
e.g.你要当心/小心他,他以前骗过你。
e.g.当心/小心地上滑。
e.g.当心/小心!前面有车。
2.小心 can be used as an adj. or adv.(like careful or carefully in English)modifying the predicate verb.but 当心 can not.
e.g.他小心地开车。(√)他当心地开车(×)
e.g.他开车很小心。(√)他开车很当心(×)