100-amp breaker wireing
Greetings and welcome!
First, regarding the 125A load center (sub panel), is that the panel rating, or the load center's main breaker (if it has one, not required if both panels are in the same structure). If the sub panel does have a main it cannot be rated higher than the breaker in the main panel, currently 60A
#6 copper is required for 60A, if you want to change to 100A you will need #3 copper. What are you planning to use that will need 100A? That is a lot of capacity (24KW!)