Hi latexdad,
The choice between Dunlop and Talalay is more a matter of preference and which one you think she would prefer more than anything else. Talalay is more lively and “springy” than Dunlop (see post #7 here). I don’t know the reasoning behind the suggestion for Dunlop as either one would work well. On the more humorous side … it may depend on how high you wanted them to bounce and on how low the ceilings were 
In most cases I tend to lean more towards firmer for young children (medium over medium or firm) but firmness is also subjective and also depends on body weight and preferences and girls tend to prefer a little softer than boys so I would probably suggest soft over medium for a young girl that was lighter or had a preference for a little softer surface because that would be firmer for a very light body weight than it would for a higher weight or for an adult. I know from my conversations with him that Shawn would probably be thinking in the same direction. For boys I would probably suggest medium over firm.
As you mentioned you can always exchange a layer if she isn’t comfortable on it and you can also add a layer down the road if necessary although if she will likely be tall and slim she likely won’t need it and may be quite comfortable on the mattress she is used to even into her teens.
Hope this helps.
Phoenix