You actually can't...unfortunately...
Basically the tickets you get are considered "COMP" tickets and can't be added to at all.
So...very unfortunately... you would have to purchase a full one day ticket (or more) for each member of your family.
It is pretty expensive - at $79 per adult and I am not sure for children - and it is an extra $52 per ticket to get the PH option added to it.
Probably not what you wanted to hear.
One option is to go all the way and purchas an Annual Pass or a Non-Expiring Park Hopper (which is more expensive, obviously) but you could use if you plan to come back - (AP if you plan to come back within the year and non-expiring if you plan to come back in a few years). That would give you the investment so that when you do come back you would not have to pay that hefty "first day" ticket again...they would all be at the "subsequent" day price. But that only works if you think you are going to be able to get back.
We knew we had to come back as soon as the trip started...so this is what we would have done.