Hey, good on you for taking up this hobby at such a young age. It's a lot of responsibility..
I hope you manage to sell your fish.. Have you tried ads on a more local fish forum? Or maybe in the local pet store?
But yeah.. maybe your mum will allow a few small heaters, if it'll keep them alive and healthy. I totally understand about not getting more tanks til you've dealt with the ones you have already.
It's a parent thing..
Your mum sounds pretty supportive of your hobby, though?
If heaters aren't possible at all, and it's not usually -that- cold in your part of Florida, maybe she'll allow a heater in a small room to run for a few days, until it warms up again?