Hi Granger,
There is no specific firmness that is “right” for every child but there are some firmness guidelines in post #2 here that should be helpful.
Their firm would be a typical firmness choice for a child because the comfort layer is in a medium firmness range (although with a 2" comfort layer and a very firm support core underneath it most adults would sink into the mattress more and would feel more of the firmness of the support core and would probably consider it to be firm) but Dunlop latex is a more supportive material than other types of foam because it has a higher compression modulus than other types of foam (compression modulus is the rate that a material becomes firmer as you compress it more deeply) so with your daughters lighter weight and because girls often prefer a little softer than boys their medium would also be a suitable choice as well even though 24 ILD is approaching the softer end of the firmness scale…
Phoenix