@Scarlet14 I'm really sorry to say this, but in your situation I would PTS. No doubt about it.
Resource guarding is difficult to manage, tends to escalate (as you've found) and, even with the best intentions, further bites are inevitable.
Right now, you can have her PTS at the time of your choosing, in a calm environment, with her family around her. She will drift off knowing that she is loved.
If she bites a visiting child you risk her being taken by the police and held in kennels, possibly for months, until she is PTS following a court order, surrounded by strangers. That would be far, far more traumatic for her. You risk prosecution and a criminal conviction. And, of course, someone will have been bitten, with the resultant impact on them.
To rehome her privately would be grossly irresponsible, and is likely to set up a cycle of repeated rehomings as owners won't cope. This will be unsettling and distressing for her, which is likely to worsen her resource-guarding.
Rehoming through a rescue, unless you can find a specialist rescue that permanently fosters dogs with a bite-history, again risks a protracted period in kennels followed by euthanasia.
I know it's incredibly unfair, but euthanasia is the least-bad outcome.