A few things to think about:
This is not your final layout. This is a project with your daughters. Someday, when they are older, you will build a more mature layout, with them, or by yourself, depending on their level of interest at that time. So there is no need to try to have everything on this one.
To you, how important is it that the trains be models? I say "models," because most toy trains are more like caricatures. Over the years a wide variety of wildly-inaccurate traditional-sized trains have been issued with paint/details suggesting many of the real-world engines. Want an L&N "Big Emma" 4-8-4? Lionel's got you covered, as long as you can look past the obvious fact that it's a NYC engine with different paint. Williams even issued a N&W J-class in two-tone blue and called it the streamlined ATSF "Blue Goose." Does stuff like that make you smile, or make you retch? Can you look at a BNSF boxcar with a roofwalk without rolling your eyes? If you need accurate models, then you need O-72 to run them on. But if you just like toy trains, then you can be happy with caricatures running on O-36 track, while your imagination does the rest.
Someone said that there is poetry in limitations. Model railroading must be the most poetic thing you can do. If you limit your curvature, you will limit your possible roster of locomotives and rolling stock, while opening up the possibilities of what you can do in your space. Smaller curves = more trains per square foot, or more operating potential per square foot. The converse is also true, of course: the wider your curves, the wider the variety of motive power, but, paradoxically, the narrower your possible uses for it.
I think it would get frustrating to have a situation where you had locos or rolling stock which could not traverse the entire layout.
Curves always look better when viewed from the inside. In your current track plan, the viewer sees the outside of the O36 curves, while the O72 curve is hidden within the layout. If you are going to have larger curves, you should take visual advantage of them by making them more prominent than the sharper ones. The pop-up in the center of the layout would allow the viewer to see the larger curves closer, and also from the inside.
FWIW, I am a toy train guy, and my own layout has O-31 minimum curves. Almost everywhere, I use O-36 or larger for the sake of appearance. The small minimum curve limits my roster to the "classic" toy trains, and items compatible with them. I'm fine with this, because I like these trains anyway, and tight turns make possible a lot of complexity which I could not do with O-72.
