"Hello dad, how are you?"
Translation:שלום אבא, מה שלומך?
Whether you use a single or double yod depends more on the spelling convention that you are using in Hebrew, rather than a change in meaning. Duo tends towards the habit of using two yods when they want to designate yod as a consonant and not as a vowel with mater lectionis. So both הי and היי can be used to approximate to any casual greeting such as hi, hey, hello, hiya, oi.