So, that said, I'm hoping for any advice concerning the choice of plans, rather than advice about teaching my children a lesson.
I'm happy to see you already found the guide. :)
My advice? Don't get a family plan, don't give a toss about trying to avoid overage costs (there aren't any in prepaid, service just stops working until you add more money), and don't modify your own usage or increase your costs to subsidize
their habits. The options provided there in the guide are cheap enough for any teenager who may actually
need a cellphone to pay for it themselves (hardware and monthly) on their own. If they can't budget their time and money on the device sufficiently to go PAYG or they actually have a need for that much mobile talk time, they can pay the price premium for a plan that doesn't meter.
The next time they ask about a phone, I'd recommend letting your children
read the guide. Tell them, "Someone has done the hard work of researching out some of the most affordable and reliable cellular providers available, and has highlighted plans that can range from about $3.50 a month up to around $45 depending on usage and need. There's also suggestions on affordable handsets ranging from feature phones to smartphones. The T-Mobile network works well in this area, so I suggest that you pay attention to Daley's mentioning of P'tel and Spot Mobile."
Basically, this addresses the need versus want issue in a way that doesn't deprive them if
they can justify the costs, and gives them the freedom and path that they could actually potentially afford on their own. They make the value call instead of you. My parents did the same with me when I was younger. "You want your own phone line? You can pay for the equipment, installation, and bills yourself." That even applied to driving and other independent right of passage stuff, and I did. It was a valuable lesson that actually left me feeling empowered in the end even if it took time to enable that freedom and groused about how other kids parents gave them what they wanted before having it myself. My parents trusted me enough to let me have my own dedicated line of communications, but instilled the cost and value of it with me. And you know what? I never missed a payment and never asked my parents for money to pay the bill those years, even after I met a girl out of country and racked up a few $1000 long distance bills... but that part's more of a boneheaded story for another time and different venue.
Point is, you know it's a want versus need issue, but you probably don't want to say no given you're asking about options to let them have what they're asking for... so don't say no, just tell them they have to fund it themselves and then provide them the tools to feed that want on their own budget. Otherwise, you may find yourself in the same trap I've seen countless other parents where they went with family plans "to save money and get more service" only to find themselves subsidizing their adult children's mobile phone habits five years after the kid has already moved out.
Anyway, enough out of my verbose soapbox going into a subject you already said you didn't want more of.
IP Daley is the expert in this, but prepare yourself for the lecture you will get about giving your teenagers smart phones. ;)
I aim to never disappoint.
Now that you're familiar with the guide and if you've got a solid internet connection, next step for your own budget is to replace that home phone line with VoIP service. ;)