We have found with our daughter that a 130 ltr SUP has been great for her learning, not too big, better in waves as well,
Phoebe also has a 115ltr JP Young Gun, this is a great board for her to blast around on, get used to footstraps and harness etc, gets on a plane better as well. It has a centre board for maintaining position if need be,
We have found that the big thing with sailing / learning for kids is the weight of the rigs, the 3.0 severne X-1 has been perfect for Phoebe, super light and not flimsy at all. She is probably nopw ready to step up to a bigger adult sail, but on lighter cruisey days she still prefers her 3.0 - once she starts water starting she will probably need a bigger sail,
check this out,