Consider the following sentences:
- 子供は本を読む。
- 母は子供[に]{LL}本を読ませる。
Since 読む is a transitive verb (i.e.: 本を読む), に is used to mark the causee (i.e.: 子供) for its causative form 読ませる.
Now, consider the following sentences (not sure whether they are correct):
- 子供は読む。
- 母は子供[を]{LL}読ませる。
Even though, 読む is a verb that accepts a direct object, it's not taking one. Therefore, を is used to mark the causee 子供 for its causative form 読ませる.
It seems that, what determines whether to choose を or に for marking the causee in a causative verb, is not the verb per se, but actually how the verb is used in the sentence. Of course, if the verb is intransitive it will be always を, but for a transitive verb it may be either に or を.
Am I missing something?
if the verb is intransitive it will be always を-- But.. 明鏡国語辞典 carries example sentences: 「代理として娘に／を行かせる」「私にも休ませてください」 ... (Sorry if I'm only confusing you..)