This is the kind of space that makes my heart race a little! I'm used to working in spaces that make the plant palette limited - dwarf this and compact that. I love the types of challenges that those spaces present, and I love making the most out of small spaces so that my client can use every last inch. But this property? I can select something that will get BIG.
The first thought that came to mind, since the client is young, newly married, and wants kids one day - is Fagus sylvatica (Copper Beech). It's purply, coppery foliage is stunning, changing colors at different times of the year. The bark is incredible, smooth in spots and as it ages, it gets wrinkly, like elephant skin. I see this as a "legacy tree," something the client can plant now, right in the middle of the yard where they can see it from the back windows, and have it age with them as their family grows. The Copper Beech will grow to 50-60' tall and 30' wide. I can imagine a super comfy chair underneat it, providing shde and relaxation for a busy family.
The client wants to attract more birds to the property, and will do so with this tree. Copper Beech provides beech nuts, an important food source for birds.
I plan to suggest it when I present the full design, and I hope she goes for it!