For those of you who live in an area with good Sprint coverage. I suggest checking out RingPlus, which is a Sprint based MVNO (Mobile Virtual Network Operator) What separates RingPlus from the other MVNOs like FreeomPop, Boost, and the like, are the amazing promotions they offer. I'm a regular visitor of the deals site, SlickDeals, and have noticed that these promotional plans come out almost every week.
Early this year I had purchased a pre-owned LG G2 from FreedomPop and was very pleased with the free plan which came with a trial for their premium voice services ($10/month) which allows for making calls through the cell towers. However if you downgrade to the free plan all of my calls were being forced through VOIP and the quality was not very good.
Anyways, the RingPlus promotion I signed up for was called "Surfing 5.40", which included 4000 units of voice, sms, and full speed LTE data. (in megabytes) The only requirement was the one time purchase of $30 of top-up balance, which is only used if there are overages on your account.
My experience with the service so far is that you are going to have some issues porting over your number and getting all of the cell tower information programmed into your phone. It took me several days and multiple phone calls to get my old number ported and for FreedomPop to release the MEID for use by another carrier. I'm still having issues (error 16) with placing calls and recieving messages over 3G/EVDO connections. My 4G/LTE connections are working great though. Some of the complications are likely due to my choice to run a custom ROM on the phone. My particular ROM is based on the Android Open Source Project (AOSP), so it does not include the utilities to automatically provision your cell tower information. I'm having to learn a lot about the guts of Android to get everything working. But that is part of the fun.
Long story shorter, I wouldn't recommend switching your main phone to RingPlus. But if you can find a compatible (Sprint) pre-owned phone for cheap, it's definitely a good way to get a good deal on your spare phone or phones for your kids.