Take the time to read the guide (
unabridged here), it'll help clear up a lot of your confusion if you let it digest and you follow definition links.
It used to be that Sprint MVNO branded handsets (Virgin, Boost) were locked into only activating on those brands and no other Sprint MVNO.
They have since changed that policy, but
it's still twitchy.
Here's Boost's requirements for carrier unlocking and porting. Generally when everything works right, for Boost and Virgin handsets, it needs to be active for a year to be able to be unlocked and/or taken elsewhere (including Sprint postpaid, IIRC).
For you, the only MVNO that would probably work would be Ting CDMA. You can check if the phones can be taken out of Boost to Ting
here.
Regarding Sprint's family plan "deal" they have going,
note the fine print. It requires paying for new handsets either on lease, contract or through their "Easy Pay" system. That's their gotcha. It's only $100/month if you pay for your handsets on a monthly basis, a price they don't include in the math. Needless to say, you're probably not going to be able to take over your Boost handsets to Sprint postpaid and get that deal. Look at your usage, do the math, and see what it'll cost to go to Ting. Nobody needs unlimited anything, and if you do that math, you'll probably realize that.
If you feel like you need to buy new handsets to get cheaper phone service, consider going with any of the other MVNOs I mentioned first. GSM handsets (AT&T, T-Mobile) are considerably cheaper than CDMA (Sprint, Verizon), even buying carrier unlocked...