Consider the following sentences:
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):
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?