A one-size-fits-all folder would be nice, but there are other options. For a total of under $50, you could create a variety of blade shapes in sizes to meet your needs.
Years ago, my son wanted to whittle (he was around 9, at the time). I decided he needed a locking folder, with a long handle and short blade. I modified an Opinel #10: shortened the blade, into a wharncliffe (with nail-nick), using to a cutting-wheel equipped Dremel and a pan of water (to cool the steel); added an easy-open handle slot with a rasp.
Years ago, my son wanted to whittle (he was around 9, at the time). I decided he needed a locking folder, with a long handle and short blade. I modified an Opinel #10: shortened the blade, into a wharncliffe (with nail-nick), using to a cutting-wheel equipped Dremel and a pan of water (to cool the steel); added an easy-open handle slot with a rasp.