Bootfitters, will look at the interaction of you and the boot. This can start by checking how much space you have inside the shell, length, heel, instep ect. They can modify the shape bigger to accomadate but not make a shell smaller, why we buy boots small not big.
They can adjust the liner, packing it out, adding foam, cuting, reshaping.
They can make custom footbeds. These are so inportant. Your foot needs to be in as neautral postion possible for the biomechanics of your leg to function correctly. Aftermarket liners such as intuition or even better foam injection liners can be fitted.
They can align the boot. Maybe just using the canting (although this is being phased out on boots) or by using shims o align the leg inside the boot. Usally done once a footbed is made.
Shell adjustments in terms of flex and upright stance can be made. Cuff adjustments can also be done in width, again usally bigger, but padding can be added to make a cuff smaller.
They will also do genrall boot work like moving/replacing buckles/ladders. Replacing Din pads, and boot damms ect ect ect.
So as you see there is lots that can be done. As I said making a small boot fit is easier then making a big boot fit, so buy small kids.