If you're looking to crunch the budget down, if you're willing to do it, you might ask if there's a cash discount, as in 'you bring cash instead of using a credit card.' It probably wouldn't be a big difference, but for sake of argument let's say it's 3% off the dive package total. Are you willing to take a big wad of cash on you in order get save that money?
Sounds like you're asking if your family can get a group rate. Nothing wrong with that. I don't know the latest recommendations on youth a max. depths; will the kids be diving like adults after certification, if things go well?
Comparing dive op.s can be an apples to oranges comparison. Here's what I'm talking about. You might dive with an op., I'll call it Op. A, that mostly uses 80-cf tanks (which IIRC have around 77.something cf at 3,000 PSI, their rated fill pressure), but can get 100-cf tanks. But 100-cf tanks tend to have a rated fill pressure of 3,300 PSI, and your tanks will likely be filled to around 3,000 PSI, so roughly around 90-cf. Still a meaningful boost, but not quite what you thought.
But let's say you compare to Aldora. Last I heard, Aldora has high-pressure 120-cf steel tanks on offer, and for example if Dad's an air hog, that could be wonderful. If he's not, those big tanks might let you guys get in 2 long dives per morning, and do other things in the afternoon (if that's what you wish to do).
Whether Aldora or Op. A (whichever that is) is the best option for your family is up to you. Just know there are differences.
And don't forget to budget for tipping boat crew. You may already know this and I don't know how much traveling you do. I try to squeeze water (value) of a rock (budget) on my dive trips, and for the cost of 4 divers (some in training) on a trip, I figure you might be, too.
Richard.