Is it $50 total, or $50 per line?
Are the phones Sprint compatible? (AT&T is a GSM network, and many phones only include the right radios to work on GSM i.e AT&T and T-Mobile.)
Personally I ended my contracts a while back and started buying GSM phones so I could choose my network. At first, I used T-Mobile, but then I switched around; I've used Straight Talk, H20, Cricket and now Google Fi.
Since you have AT&T compatible phones, you could switch to Cricket. You could get 3 x $40/month plans - $30 family discount = $90 / month total.
Cricket isn't perfect, and it's actually part of AT&T, but it is one option for you. With a little research, you could do really well with H20 wireless or another AT&T compatible MVNO.