I would start with 50mg if you are combining it for the first time. Also, when redosing or when you are starting your new day taking other depressants, keep in mind that it has a very long half life. If I'm not mistaken, it's about 20 hours for adults, for kids, it's about half of this.
When you combine Hydroxyzine with opiates, you will caych a nod much more easily and you will sleep without waking up at all till the morning.
Take it as seriously as you would with benzodiazepines.