A 125 is fine for skinny school boys but I suspect you’ll want something with more grunt. You can detune the big bikes but they are still hard to kick so you must take good leg bad leg into consideration. As pointed out Betas kick on the correct side while all the others, well I don’t know what they were thinking.?
Best thing is to try a few. If you can get to a local event there are always bikes for sale. If you haven’t tried a trials bike be prepared to be surprised. They respond instantly to input. I’ve watched a lot of experianced riders flip them thinking they’re weak little toys.