In a general way island layouts have more room for scenery/buildings at the cost of tighter curves and access/reach problems. Around-the-walls layouts can have broader curves and no access problems at the expense of less scenery and buildings. I am happy with an around-the-walls "shelf" pike because I am more interested in operating a miniature railroad than I am in creating a miniature World. For me scenery and structures are the stage in support of the actors, the trains. I suspect for the OP and his two young daughters the miniature World is the attraction and the trains will add animation.
I do think access is a real problem whether the reach is 36" or 39", most especially for the two young model railroaders.
Several pics of my......somewhat spartan Pike. Looking West:
My helper occupying the futon. 
The Southeast corner:
The Southwest corner:
The Northeast corner:
