For one line? Share Everything for one line with 2 GB of data is $100/mo. Add three basic phones and it's $190/mo. for four lines. While I can't speak for the other three lines on your plan, I can't think of a good reason to enter a new two year agreement with a dumb phone.
I'm guessing that if you've got an Alltel grandfathered plan you're currently month-to-month, if you don't have a contract it's definitely worth looking prepaid, especially for the three dumb lines, where the upfront cost is peanuts since those phones are so inexpensive. Pick up a cheap phone and a cheap plan and they're set. For your mother, it's still worth looking into, something like Straight Talk could be good, but the initial cost of a phone will be greater since you'd be buying one outright instead of getting a phone with an initial discount from Verizon.
Calling Verizon is definitely good advice, see what they can offer you coming from Alltel. If you're not on contract, you may want to mention that you've seen good deals from other providers, see if they're willing to offer something. Your mileage may vary.