While any of those providers may cover your area, there will be regions where signal strength is not as good... and if one of those poor signal areas happen to be a place that you frequent (such as your home/office/etc) then it would become frustrating very quickly.
So, ask people who work with you what provider they have and what sort of reception they get at/near the office (see if there is a pattern). While phone type also makes a difference, if you find that all/most of coworkers who have one provider have weak signal at the office, it could just be a weak spot for that provider.
Similarly, do you have friends with cell phones that come to your house? If so, ask what provider they have and see how strong their cell reception is (how many bars show up on their phone).
As any example, i live in the washington dc area. those 4 same providers also have a presence around here and say they "have coverage in my area."
but sprint and tmobiles signals are so weak at my house that any friends who come over to my house (in the suburbs) can not use their service at all. verizon on the other hand gets "okay" service at my house. and at&t gets good reception at my house.
conversely, at my old office, sprint and tmobile and at&t were okay while verizon was strong. and also (if i used the subway around here this would be a huge factor) verizon has antennas in the subway tunnels making them the only provider in DC that works on the subway when its at underground stations (though i don't use the subways often enough for that to be a deciding factor for me - but just explaining that this is the sort of thing to think about in terms of where do you spend a lot of your time, etc).
Aside from signal strength, make the decision if you want a dataplan or just phone or just phone and text
(though, warning, you said you have two junior high kids - make sure their expectations on what and how much they're allowed to use is determined -- if their friends text a lot or send pictures etc then it will mount up very quickly)... because the thing with data and/or text, it's all "available" on the PHONE even if not part of your "service plan," thus it can still get used and becomes an extra charge on your next bill. if it's just a once in a blue moon sort of thing, then no big deal but at a certain point if it gets used a lot it would be cheaper to consider having it as part of the service plan.
but deciding phone only, or also text, or also data changes the costs and phone types you are looking at.
having said all that ...
my family has at&t's family plan. we don't use data service at ALL (though we do use text messages once in a while even though it isn't a prepaid part of our service plan, but our level of use on them is not enough to justify upgrading the plan to include text).
we chose it because of the providers it has the strongest reception by our house (else we would get very frustrated if we had dropped calls at a location where we spend a lot of time). also in-network calls (to other at&t cell users in the US) don't use up minutes on our plan and as it happens we have a lot of other relatives who also use at&t so that worked out as a little side bonus for us.
We've never really had any complaints about the service, but then, we don't really expect much of the service as it is just "a phone" to us.
We did try verizon for a year in between as they were offering us a better rate. but for our neighborhood the reception was just okay. and on top of that, when it came time to renew, our price jumped up (for some reason that i don't fully understand as i was not part of that process) to a level that we considered unreasonable, so we switched back to at&t -- mind you, that was 10(-ish) years ago at this point, i'm sure their plans and policies are much different now.
As a general point, I'll add in ...start with a conservative plan (in terms of what you think you'll need for minutes and so on) and then upgrade if you find it's not enough... because (generally) it's easier to get an upgrade on your service if you need more than it is to get a downgrade if you're over paying.
(Of course, clarify before signing "what is the process if i decide two months from now that i want to increase my plan?" in case there is some specific that would make upgrading more difficult)
as someone who has kids to take care of, i'll also toss in the usual soapbox speech that i give to my brother for his kids ...if this is the kids first personal phone, make sure they remember it's not a toy (just because it's there next to you all the time doesn't mean you have to use/fidget with it all the time), to take care of it (what are the house rules if they damage or lose the phone - do they pay the repair/replacement themselves, etc), no talking on the phone while driving/biking/skateboarding/etc, not to be rude on the phone when other people are talking, and so on.
this turned in to a really long ramble -- sorry -- but hopefully there was some useful tidbit of info in there, somewhere.
