If you round off the pointy corners it will help disguise the shape of the garden and give you a greater depth of border to build up a tropical jungly feel if that is what you want. You would need to be extemely careful with bamboo as many kinds spread and can be very hard to dig out and the neighbours might not want them as much as you do! It would be safer to try for a similar effect with Miscanthus grasses, some of which grow really large but are more manageable. If you have rather boggy clay you could grow some Hostas. 'Sum and Substance' is big, but if you want something really large look for 'Empress Wu'. Another good jungly plant is Darmera Peltata, that also enjoys moist soil. My daughter used to hide among its stems when she was little, I'm sure your 4 yr old would like it too! Just a few things to whet your appetite - I'm sure others will be along soon with more ideas!
Digging out stumps is warm work for the winter, too, and quite rewarding. You can then leave the stumps in a corner for the minibeasts to live in. Four-year-olds like them, too!
If you cut the grass at least weekly through the summer it'll be respectable; twice a week it'll be the best in the road! Weed/feed/seed alslo helps but mowing is the main thing!
The building work won't be for a year or two yet but the builders are family so I trust them to take care.
My garden will be mainly used to entertain and relax in. I plan to include a outdoor kitchen and/or bar and different seating areas to take advantage of the available sun/shade. I would like a less traditional feel to it and I prefer plants along the lines of palms and bamboo over the more traditional English flowering plants. I do need a lawn area for now as I have a 4yr old but not as much as I have now